A word of warning here - if the pdf is generated as an image, none of these techniqes will help. The only thing the MSAA or COM apis will give you is the page image which contains all the text you want to check.
Of course, being able to offer everyone else's suggestions, then add that if the application you're testing is generating its pdf files as images, you can't test them without an OCR plugin will give you extra bonus points on your interviews!
I appreciate more information. For now I was looking for the answer to a possible interview question. I guess that my answer would now be something such as "I investigated this on SQAForums. I would need to go back to my thread on the topic. But I have a very strong starting point."