Use ghostscript to compress PDF output on RTD

.readthedocs.yaml

@@ -11,6 +11,9 @@ build:
     python: "3.11"
   apt_packages:
     - graphviz
+    - ghostscript
+  jobs:
+    post_build: ./scripts/rtd-compress-pdf.sh
 
 formats:
   - pdf

scripts/rtd-compress-pdf.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env sh
+
+# Compresses the PDF output using ghostscript on Read the Docs.
+
+OUTPUT_FILENAME="gm0.pdf"
+DPI=300
+PDF_DIR="$READTHEDOCS_OUTPUT/pdf"
+
+# Check if the PDF build is enabled based on if the PDF output directory exists
+if [ -d "$PDF_DIR" ]; then
+    existing_pdf=$(ls "$PDF_DIR")
+
+    # actually compress the PDF
+    gs -sDEVICE=pdfwrite \
+        -dCompatibilityLevel=1.4 \
+        -dPDFSETTINGS=/ebook \
+        -dNOPAUSE \
+        -dBATCH -dColorImageResolution="$DPI" \
+        -sOutputFile="$PDF_DIR/$OUTPUT_FILENAME" "$PDF_DIR/$existing_pdf"
+
+    # readthedocs expects only one PDF output in the directory
+    # see https://docs.readthedocs.io/en/stable/reference/environment-variables.html#envvar-READTHEDOCS_OUTPUT
+    rm "$PDF_DIR/$existing_pdf"
+fi