Does PDF conversion fail?
Did PDF conversion fail when using 'Print' or 'Export PDF' function to save PDF?
☝ Please check the security of the attachment.
If a secure file is attached, the document cannot be converted to a full PDF.
PDF conversion is not possible even if some of the contents are secured as well as the security of the file itself.
✌ Please check if the document capacity written in Google form exceeds 10mb.
When you create a document in Google form, Google restricts PDF conversion when the capacity exceeds 10mb.
The document does not translate the body of the document, but only the approval line is converted screen.