diff --git a/crates/engine/src/compare.rs b/crates/engine/src/compare.rs
--- a/crates/engine/src/compare.rs
+++ b/crates/engine/src/compare.rs
@@ -60,7 +60,17 @@ impl Session {
         let b = self.get(new).ok_or(EditError::NoDocument)?;
         a.check_copying()?;
         b.check_copying()?;
-        Ok(pdfcraft_compare::compare(&a.words(), &b.words()))
+        let (old_words, new_words) = (a.words(), b.words());
+        // A scanned page is a picture: with no text to read, "no changes" would be a false answer.
+        for (d, words) in [(a, &old_words), (b, &new_words)] {
+            if words.is_empty() {
+                return Err(EditError::Invalid(format!(
+                    "{} has no text to compare (it may be a scan). Run Recognize text on it first, or use the visual compare.",
+                    d.name
+                )));
+            }
+        }
+        Ok(pdfcraft_compare::compare(&old_words, &new_words))
     }
 
     /// Visual compare: regions where page n of `new` looks different from page n of `old`
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -5196,6 +5196,20 @@ fn detecting_comb_boxes_adds_one_comb_field_per_row() {
     assert_eq!(d.can_undo(), Some("Detect form fields"));
 }
 
+#[test]
+fn comparing_a_document_without_text_says_so_instead_of_reporting_no_changes() {
+    let mut s = Session::new().with_clock(|| 1_700_000_000);
+    let text = s.create_from_text("t", "The contract starts on Monday.").unwrap();
+    let scan = s.create_blank(200.0, 300.0, 1).unwrap();
+    let text = s.open("text.pdf", None, text, None).unwrap();
+    let scan = s.open("scan.pdf", None, scan, None).unwrap();
+    for (old, new) in [(text, scan), (scan, text), (scan, scan)] {
+        let Err(EditError::Invalid(m)) = s.compare(old, new) else { panic!("a document without text is refused") };
+        assert!(m.contains("scan.pdf has no text to compare") && m.contains("Recognize text"), "{m}");
+    }
+    assert!(s.compare(text, text).is_ok());
+}
+
 #[test]
 fn comparing_two_versions_of_a_document() {
     let mut s = Session::new().with_clock(|| 1_700_000_000);
@@ -8720,69 +8734,69 @@ fn pdf_with_annots(annots: &[&[u8]]) -> Vec<u8> {
     bytes
 }
 
-/// ISO 32000-2 Table 224 (`/NeedAppearances`) and Table 192 (`/MK /CA`): a form that leaves its
-/// appearances to the viewer shows its check boxes and radio buttons drawn from `/MK`, over the
-/// filled squares the file carries: a checked box its check mark (ZapfDingbats `4`), an unchecked
-/// one its empty border, a radio button its dot (`l`), on screen and printed as an image. Without
-/// `/NeedAppearances` the file's own squares show. Saving writes the file as it was.
-#[test]
-fn need_appearances_buttons_are_displayed_from_their_captions() {
-    let pdf = |need: bool| {
-        let flag = if need { "/NeedAppearances true " } else { "" };
-        format!(
-            "%PDF-1.7\n\
-             1 0 obj << /Type /Catalog /Pages 2 0 R /AcroForm 4 0 R >> endobj\n\
-             2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 >> endobj\n\
-             3 0 obj << /Type /Page /Parent 2 0 R /MediaBox [0 0 400 200] /Annots [6 0 R 7 0 R 8 0 R] >> endobj\n\
-             4 0 obj << /Fields [6 0 R 7 0 R 8 0 R] {flag}/DA (/Helv 0 Tf 0 g) /DR << /Font << /Helv 5 0 R >> >> >> endobj\n\
-             5 0 obj << /Type /Font /Subtype /Type1 /BaseFont /Helvetica /Encoding /WinAnsiEncoding >> endobj\n\
-             6 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /T (on) /V /Yes /AS /Yes /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (4) >> /Rect [20 100 60 140] /F 4 /P 3 0 R /AP << /N << /Yes 9 0 R /Off 9 0 R >> >> >> endobj\n\
-             7 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /T (off) /V /Off /AS /Off /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (4) >> /Rect [100 100 140 140] /F 4 /P 3 0 R /AP << /N << /Yes 9 0 R /Off 9 0 R >> >> >> endobj\n\
-             8 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /Ff 32768 /T (radio) /V /B /AS /B /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (l) >> /Rect [180 100 220 140] /F 4 /P 3 0 R /AP << /N << /B 9 0 R /Off 9 0 R >> >> >> endobj\n\
-             9 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 40 40] /Length 18 >> stream\n0 g 4 4 32 32 re f\nendstream endobj\n\
-             trailer << /Root 1 0 R >>\n%%EOF"
-        )
-        .into_bytes()
-    };
-    let render = |bytes: Arc<Vec<u8>>| {
-        let mut r = pdfcraft_render::PageRenderer::new(bytes, pdfcraft_render::RenderConfig::default());
-        r.render(pdfcraft_render::RenderRequest { page: 0, scale: 1.0, ..Default::default() })
-    };
-    // Dark pixels of the page (drawn at scale 1, 400 x 200) in the user-space box x0..x1, y0..y1.
-    let ink = |page: &pdfcraft_render::RenderedPage, (x0, y0, x1, y1): (u32, u32, u32, u32)| {
-        let (px, _) = page.rgba.as_chunks::<4>();
-        (200 - y1..200 - y0)
-            .flat_map(|y| (x0..x1).map(move |x| (x, y)))
-            .filter(|&(x, y)| px.get((y * page.width + x) as usize).is_some_and(|p| p[0] < 128 && p[1] < 128 && p[2] < 128))
-            .count()
-    };
-    // In a widget whose lower-left corner is (x, y): its whole inside, the top-left of its inside
-    // (where a check mark leaves the box empty), and the lower-left corner (outside a dot).
-    let inside = |x: u32, y: u32| (x + 6, y + 6, x + 34, y + 34);
-    let top_left = |x: u32, y: u32| (x + 6, y + 26, x + 14, y + 34);
-    let corner = |x: u32, y: u32| (x + 5, y + 5, x + 9, y + 9);
-    let mut s = Session::new();
-    let id = s.open("form.pdf", None, Arc::new(pdf(true)), None).unwrap();
-    let shown = render(s.get(id).unwrap().display.clone());
-    let raster = pdfcraft_print::Raster { dpi: 72.0, grayscale: false };
-    let pictures = crate::print_raster::rasterize(s.get(id).unwrap(), &[0], &raster, pdfcraft_print::Content::Document).unwrap();
-    let printed = render(Arc::new(pictures));
-    for (page, how) in [(&shown, "shown"), (&printed, "printed")] {
-        let (check, check_corner) = (ink(page, inside(20, 100)), ink(page, top_left(20, 100)));
-        assert!(check > 100 && check < 600, "{how}: a check mark, not the 28 x 28 square: {check}");
-        assert_eq!(check_corner, 0, "{how}: the check mark leaves the top left empty");
-        assert_eq!(ink(page, inside(100, 100)), 0, "{how}: the unchecked box is empty");
-        assert!(ink(page, (100, 100, 140, 102)) > 30, "{how}: and keeps its border");
-        let dot = ink(page, (188, 108, 212, 132));
-        assert!(dot > 300 && ink(page, corner(180, 100)) == 0, "{how}: the radio button's dot: {dot}");
-    }
-    let saved = s.save_bytes(id).unwrap();
-    assert!(saved.windows(18).any(|w| w == b"0 g 4 4 32 32 re f"), "the file keeps its own appearances");
-
-    // Without /NeedAppearances the file's filled squares are shown.
-    let id = s.open("plain.pdf", None, Arc::new(pdf(false)), None).unwrap();
-    let shown = render(s.get(id).unwrap().display.clone());
-    for x in [20, 100, 180] {
-        assert!(ink(&shown, inside(x, 100)) > 700, "the file's square at {x}");
-    }
-}
+/// ISO 32000-2 Table 224 (`/NeedAppearances`) and Table 192 (`/MK /CA`): a form that leaves its
+/// appearances to the viewer shows its check boxes and radio buttons drawn from `/MK`, over the
+/// filled squares the file carries: a checked box its check mark (ZapfDingbats `4`), an unchecked
+/// one its empty border, a radio button its dot (`l`), on screen and printed as an image. Without
+/// `/NeedAppearances` the file's own squares show. Saving writes the file as it was.
+#[test]
+fn need_appearances_buttons_are_displayed_from_their_captions() {
+    let pdf = |need: bool| {
+        let flag = if need { "/NeedAppearances true " } else { "" };
+        format!(
+            "%PDF-1.7\n\
+             1 0 obj << /Type /Catalog /Pages 2 0 R /AcroForm 4 0 R >> endobj\n\
+             2 0 obj << /Type /Pages /Kids [3 0 R] /Count 1 >> endobj\n\
+             3 0 obj << /Type /Page /Parent 2 0 R /MediaBox [0 0 400 200] /Annots [6 0 R 7 0 R 8 0 R] >> endobj\n\
+             4 0 obj << /Fields [6 0 R 7 0 R 8 0 R] {flag}/DA (/Helv 0 Tf 0 g) /DR << /Font << /Helv 5 0 R >> >> >> endobj\n\
+             5 0 obj << /Type /Font /Subtype /Type1 /BaseFont /Helvetica /Encoding /WinAnsiEncoding >> endobj\n\
+             6 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /T (on) /V /Yes /AS /Yes /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (4) >> /Rect [20 100 60 140] /F 4 /P 3 0 R /AP << /N << /Yes 9 0 R /Off 9 0 R >> >> >> endobj\n\
+             7 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /T (off) /V /Off /AS /Off /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (4) >> /Rect [100 100 140 140] /F 4 /P 3 0 R /AP << /N << /Yes 9 0 R /Off 9 0 R >> >> >> endobj\n\
+             8 0 obj << /Type /Annot /Subtype /Widget /FT /Btn /Ff 32768 /T (radio) /V /B /AS /B /DA (/Helv 0 Tf 0 g) /MK << /BC [0 0 0] /CA (l) >> /Rect [180 100 220 140] /F 4 /P 3 0 R /AP << /N << /B 9 0 R /Off 9 0 R >> >> >> endobj\n\
+             9 0 obj << /Type /XObject /Subtype /Form /BBox [0 0 40 40] /Length 18 >> stream\n0 g 4 4 32 32 re f\nendstream endobj\n\
+             trailer << /Root 1 0 R >>\n%%EOF"
+        )
+        .into_bytes()
+    };
+    let render = |bytes: Arc<Vec<u8>>| {
+        let mut r = pdfcraft_render::PageRenderer::new(bytes, pdfcraft_render::RenderConfig::default());
+        r.render(pdfcraft_render::RenderRequest { page: 0, scale: 1.0, ..Default::default() })
+    };
+    // Dark pixels of the page (drawn at scale 1, 400 x 200) in the user-space box x0..x1, y0..y1.
+    let ink = |page: &pdfcraft_render::RenderedPage, (x0, y0, x1, y1): (u32, u32, u32, u32)| {
+        let (px, _) = page.rgba.as_chunks::<4>();
+        (200 - y1..200 - y0)
+            .flat_map(|y| (x0..x1).map(move |x| (x, y)))
+            .filter(|&(x, y)| px.get((y * page.width + x) as usize).is_some_and(|p| p[0] < 128 && p[1] < 128 && p[2] < 128))
+            .count()
+    };
+    // In a widget whose lower-left corner is (x, y): its whole inside, the top-left of its inside
+    // (where a check mark leaves the box empty), and the lower-left corner (outside a dot).
+    let inside = |x: u32, y: u32| (x + 6, y + 6, x + 34, y + 34);
+    let top_left = |x: u32, y: u32| (x + 6, y + 26, x + 14, y + 34);
+    let corner = |x: u32, y: u32| (x + 5, y + 5, x + 9, y + 9);
+    let mut s = Session::new();
+    let id = s.open("form.pdf", None, Arc::new(pdf(true)), None).unwrap();
+    let shown = render(s.get(id).unwrap().display.clone());
+    let raster = pdfcraft_print::Raster { dpi: 72.0, grayscale: false };
+    let pictures = crate::print_raster::rasterize(s.get(id).unwrap(), &[0], &raster, pdfcraft_print::Content::Document).unwrap();
+    let printed = render(Arc::new(pictures));
+    for (page, how) in [(&shown, "shown"), (&printed, "printed")] {
+        let (check, check_corner) = (ink(page, inside(20, 100)), ink(page, top_left(20, 100)));
+        assert!(check > 100 && check < 600, "{how}: a check mark, not the 28 x 28 square: {check}");
+        assert_eq!(check_corner, 0, "{how}: the check mark leaves the top left empty");
+        assert_eq!(ink(page, inside(100, 100)), 0, "{how}: the unchecked box is empty");
+        assert!(ink(page, (100, 100, 140, 102)) > 30, "{how}: and keeps its border");
+        let dot = ink(page, (188, 108, 212, 132));
+        assert!(dot > 300 && ink(page, corner(180, 100)) == 0, "{how}: the radio button's dot: {dot}");
+    }
+    let saved = s.save_bytes(id).unwrap();
+    assert!(saved.windows(18).any(|w| w == b"0 g 4 4 32 32 re f"), "the file keeps its own appearances");
+
+    // Without /NeedAppearances the file's filled squares are shown.
+    let id = s.open("plain.pdf", None, Arc::new(pdf(false)), None).unwrap();
+    let shown = render(s.get(id).unwrap().display.clone());
+    for x in [20, 100, 180] {
+        assert!(ink(&shown, inside(x, 100)) > 700, "the file's square at {x}");
+    }
+}
diff --git a/crates/ui-egui/tests/it/view.rs b/crates/ui-egui/tests/it/view.rs
--- a/crates/ui-egui/tests/it/view.rs
+++ b/crates/ui-egui/tests/it/view.rs
@@ -16,6 +16,21 @@ const PAGES: &[u8] = b"%PDF-1.7
 trailer << /Root 1 0 R >>
 %%EOF";
 
+/// [`PAGES`] with a line of text on page 2: compare refuses a document that has none.
+const PAGES_WITH_TEXT: &[u8] = b"%PDF-1.7
+1 0 obj << /Type /Catalog /Pages 2 0 R >> endobj
+2 0 obj << /Type /Pages /Kids [3 0 R 4 0 R 5 0 R] /Count 3 /MediaBox [0 0 300 400] /Resources << /Font << /F1 7 0 R >> >> >> endobj
+3 0 obj << /Type /Page /Parent 2 0 R /Annots [6 0 R] >> endobj
+4 0 obj << /Type /Page /Parent 2 0 R /Contents 8 0 R >> endobj
+5 0 obj << /Type /Page /Parent 2 0 R >> endobj
+6 0 obj << /Type /Annot /Subtype /Link /Rect [50 300 250 350] /Border [0 0 0] /Dest [5 0 R /Fit] >> endobj
+7 0 obj << /Type /Font /Subtype /Type1 /BaseFont /Helvetica >> endobj
+8 0 obj << /Length 44 >> stream
+BT /F1 12 Tf 40 300 Td (Hello compare) Tj ET
+endstream endobj
+trailer << /Root 1 0 R >>
+%%EOF";
+
 use crate::gpu;
 
 fn harness(options: &'static [(&'static str, &'static str)]) -> Harness<'static, PdfCraftApp> {
@@ -185,8 +200,8 @@ fn page_gaps_comparison_harness() -> Harness<'static, PdfCraftApp> {
         let mut app = PdfCraftApp::new();
         app.set_option("language", "en").unwrap();
         app.set_option("left", "closed").unwrap();
-        app.open_bytes("old.pdf", None, PAGES.to_vec()).unwrap();
-        app.open_bytes("new.pdf", None, PAGES.to_vec()).unwrap();
+        app.open_bytes("old.pdf", None, PAGES_WITH_TEXT.to_vec()).unwrap();
+        app.open_bytes("new.pdf", None, PAGES_WITH_TEXT.to_vec()).unwrap();
         app.set_option("zoom", "50").unwrap();
         app.compare_old = Some(app.views[0].id);
         app.run_compare();
@@ -222,7 +237,7 @@ fn page_gaps_comparison_control_preserves_both_states() {
         h.run_steps(4);
         assert!(h.state().views.iter().all(|v| v.show_page_gaps == show), "comparison rejected page-gaps={value}");
     }
-    h.state_mut().open_bytes("unrelated.pdf", None, PAGES.to_vec()).unwrap();
+    h.state_mut().open_bytes("unrelated.pdf", None, PAGES_WITH_TEXT.to_vec()).unwrap();
     h.state_mut().set_option("page-gaps", "off").unwrap();
     assert!(h.state().views[0].show_page_gaps && h.state().views[1].show_page_gaps);
     assert!(!h.state().views[2].show_page_gaps);