PDF documents are often public facing and organizations need to trust that these are generated reliably, they are correct, and that the intended readers can open them and see what they need to see or fill out the necessary information.
Security is also a big concern. You don't want your PDFs to be vulnerable to slip bombs. Finally, you want to ensure that your PDFs to be compatible with multiple vendors, applications, browsers and devices.
When testing PDF documents, it is important to test the PDF validity, layout and data. PDF validity tests whether the PDF is generated correctly and with the right content. PDF layout ensures that the documents looks as per requirements and PDF data verifies that the document was created as per the tags that surround the content on the document.
Other PDF tests to consider include verifying the content of the PDF and whether the PDF downloads.