fix(pdf): Flate-compress the exported PDF streams

printpdf 0.9's `PdfSaveOptions::optimize` is a no-op — its `doc.compress()`
call is commented out — so every page's content stream (megabytes of vector
operators for a CAD sheet) was written uncompressed: a 16-sheet drawing set
exported as 280 MB. Serialise through `printpdf::to_lopdf_doc` and run
lopdf's `compress()` before writing; the same export is now 49 MB and
readers see ordinary FlateDecode streams.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -38,6 +38,7 @@ dependencies = [
  "js-sys",
  "libloading",
  "log",
+ "lopdf",
  "lyon_tessellation",
  "meshopt",
  "naga 27.0.3",

Cargo.toml

@@ -93,6 +93,8 @@ i18n-embed = { version = "0.16", features = ["desktop-requester"] }
 rayon = "1"
 open = "5"
 printpdf = { version = "0.9.1", default-features = false, features = ["images"] }
+# The lopdf printpdf builds on: used to Flate-compress the exported PDF streams.
+lopdf = { version = "0.39.0", default-features = false, features = ["time"] }
 ureq = { version = "3", default-features = false, features = ["rustls", "platform-verifier", "win-system-proxy"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]

src/io/pdf_export.rs

@@ -424,7 +424,16 @@ fn build_pdf_pages(pages: &[PdfPageInput], plot_style: Option<&PlotStyleTable>)
             .map_err(|error| format!("Page {}: {error}", index + 1))?;
     }
     let mut warnings = Vec::new();
-    Ok(doc.save(&PdfSaveOptions::default(), &mut warnings))
+    // printpdf 0.9's `optimize` is a no-op (its `doc.compress()` is commented
+    // out), so every page's content stream — megabytes of vector operators
+    // for a CAD sheet — went out uncompressed: a 16-sheet set weighed 280 MB.
+    // Serialise through lopdf ourselves and Flate-compress the streams.
+    let mut lo = printpdf::to_lopdf_doc(&doc, &PdfSaveOptions::default(), &mut warnings);
+    lo.compress();
+    let mut bytes = Vec::new();
+    lo.save_to(&mut bytes)
+        .map_err(|error| format!("PDF serialisation failed: {error}"))?;
+    Ok(bytes)
 }
 
 #[cfg(not(target_arch = "wasm32"))]