What does the "Validate PDF" function check for?

There are a number of checks made on the PDF. It checks for improper attachments such as media or viruses. It checks for a maximum size of 14 inches by 14 inches and for non standard size outside of 'Letter' and 'A4'. It checks for non-embedded fonts and if the font is licensed. It checks to see if the attached file is a valid PDF, ASCII text or PCT-EASY zip file.

USPTO FAQ - Questions and Answers (2007JUN01)