Temporarily disable time-consuming PDF building

Signed-off-by: Arthit Suriyawongkul <arthit@gmail.com>

.github/workflows/validate_pull_request.yml

@@ -29,6 +29,8 @@ jobs:
         run: pip install -r spec-parser/requirements.txt
       - name: Build model
         run: python3 spec-parser/main.py spdx-3-model/model spdx-spec/docs/model
+      - name: Disable PDF build
+        run: sed -i '/- pdf-export:/,+1 d' spdx-spec/mkdocs.yml  # Temporarily disable time-consuming PDF build
       - name: Build docs
         run: mkdocs build -v --clean
         working-directory: spdx-spec