Actual fix

src/cff/dict/font_dict.rs

@@ -68,6 +68,7 @@ pub fn rewrite_font_dict_index(
     offsets: &mut Offsets,
     metadata: &CIDMetadata,
     w: &mut Writer,
+    top_dict_is_missing_font_matrix: bool,
 ) -> Result<()> {
     let mut dicts = vec![];
 
@@ -77,15 +78,34 @@ pub fn rewrite_font_dict_index(
         let dict = metadata.font_dicts.get(old_df as usize).ok_or(SubsetError)?;
         let mut w = Writer::new();
 
-        // See comment in `generate_font_dict_index`.
-        w.write(dict.font_matrix.as_ref().unwrap_or(&[
-            Number::one(),
-            Number::zero(),
-            Number::zero(),
-            Number::one(),
-            Number::zero(),
-            Number::zero(),
-        ]));
+        // See comment in `rewrite_top_dict_index`.
+        w.write(
+            dict.font_matrix
+                .map(|m| {
+                    if top_dict_is_missing_font_matrix {
+                        [
+                            // TODO: Is that the proper way to scale x1000? But all font matrices only seem to have
+                            // a scale, anyway
+                            Number::from_f32((m[0].as_f64() * 1000.0) as f32),
+                            Number::zero(),
+                            Number::zero(),
+                            Number::from_f32((m[3].as_f64() * 1000.0) as f32),
+                            Number::zero(),
+                            Number::zero(),
+                        ]
+                    } else {
+                        m
+                    }
+                })
+                .unwrap_or([
+                    Number::one(),
+                    Number::zero(),
+                    Number::zero(),
+                    Number::one(),
+                    Number::zero(),
+                    Number::zero(),
+                ]),
+        );
         w.write(FONT_MATRIX);
 
         // Write the length and offset of the private dict.
@@ -126,8 +146,7 @@ pub fn rewrite_font_dict_index(
 pub fn generate_font_dict_index(offsets: &mut Offsets, w: &mut Writer) -> Result<()> {
     let mut sub_w = Writer::new();
 
-    // Similarly to ghostscript, write a default font matrix. Fixes issues for some printers
-    // https://leahneukirchen.org/blog/archive/2022/10/50-blank-pages-or-black-box-debugging-of-pdf-rendering-in-printers.html
+    // See comment in `rewrite_top_dict_index`
     sub_w.write([
         Number::one(),
         Number::zero(),

src/cff/dict/top_dict.rs

@@ -106,7 +106,12 @@ pub fn rewrite_top_dict_index(
         sub_w.write(FONT_NAME);
     }
 
-    // Write a default font matrix, if it does not exist.
+    // See https://bugs.ghostscript.com/show_bug.cgi?id=690724#c12 and https://leahneukirchen.org/blog/archive/2022/10/50-blank-pages-or-black-box-debugging-of-pdf-rendering-in-printers.html
+    // We assume that if at least one font dict has a matrix, all of them do.
+    // Case 1: Top DICT MATRIX is some, FONT DICT MATRIX is some -> supplied, supplied
+    // Case 2: Top DICT MATRIX is none, FONT DICT MATRIX is some -> (0.001, 0, 0, 0.001, 0, 0), supplied * 1000
+    // Case 3: Top DICT MATRIX is some, FONT DICT MATRIX is none -> supplied, (1, 0, 0, 1, 0, 0)
+    // Case 4: Top DICT MATRIX is none, FONT DICT MATRIX is none -> (0.001, 0, 0, 0.001, 0 0), (1, 0, 0, 1, 0, 0)
     sub_w.write(top_dict_data.font_matrix.as_ref().unwrap_or(&[
         Number::from_f32(0.001),
         Number::zero(),

src/cff/mod.rs

@@ -199,6 +199,7 @@ pub fn subset(ctx: &mut Context<'_>) -> Result<()> {
                 &mut offsets,
                 cid_metadata,
                 &mut w,
+                table.top_dict_data.font_matrix.is_none(),
             )?,
         }