fix: Reset base output stream position before PDF file reading to ensure correct parsing.

models/sign_document.py

@@ -33,6 +33,7 @@ class SignDocument(models.Model):
         # 3. Load the base output into PdfFileReader
         # base_output is a BytesIO
         try:
+            base_output.seek(0)  # IMPORTANT: Reset stream position to start
             base_pdf = PdfFileReader(base_output, strict=False)
         except (ValueError, PdfReadError):
             # Should not happen if super succeeded, but safety first