Already own the pdf! can i import?

Im just wondering if since i already own the pdfs can i just import them  instead of having to buy them again through roll20

Yes, personal use is not a copyright infringement. 

There isn't a way to just import a pdf, no.  The various books available through Roll20 are integrated into the platform. You can copy/paste text from the pdf when building a character or to create handouts (for rules, equipment, abilities, etc.) to have them for reference in game.

I'm not sure that answers the question. If you buy them through roll20 you get different products . You get maps, character sheets, tokens, handouts, etc., all formatted for roll20. With the pdf, you can import them, sort of, but they will just be your pdf files- you'll still have to set up maps, characters, etc. 

Ah, I realize that I was too hasty in my answer.  You can use products you own for personal use.  I have used PDF's in my own game.  However you have to somehow get the maps into a format that you can place on a map layer.  I think I have used the snip tool in the past to make a jpg.  But honestly, it's been a while and I don't remember the exact way I did it.  You can drop pdf pages into handouts as the image though.

I think you can upload pdfs as images to roll20, handy for importing maps, or to create full-page handouts, but not for much else.