If you just want to store the PDFs in the database, that's simple enough (better would be to store them somewhere on the filesystem, and store their filenames in the database.)
If you want to store a bunch of hand-picked keywords to search these PDFs on, that's not much difficult. Each PDF record in the document table would (naturally) have a second table, with three columns, the first a unique ID, the second will contain IDs that match those in the PDF table, and the third contains a keyword. (Yes, I know, this could be better normalised...)
Find all PDFs that have the keyword "foo" attached:
SELECT pdffile from pdfs, keywords WHERE pdfs.id=keywords.pdfid AND keywords.keyword='foo'
If you actually want to search the content of the PDFs, that's another story - you'll need to know how to read them. Adobe supply a PDF manual, of which a copy appears to be available here - if not, presumably you can get it ad Adobe's site; either way, it's over 3MB. You probably won't need most of it, but you won't yet know which bits you will...