Hi Sam. You can import PDF's into the image section of a Handout. Unfortunately, you can only import 1 page of the PDF at a time, and each page will have to be on a separate Handout. You can link handouts together by including the handout's name within single brackets [Foo], you could create a "table of contents" handout that links to each page. Not exactly efficient.
You may be able to export(or extract images from your PDF) and import them as images to your library. You might also be able to copy text from the PDF and paste it into a Handout or other text area such as the GM notes or Journal Bio. Screen shots may be another option to create images for import as well.
I usually keep a PDF open outside of roll20 for reference and only import maps or important images as needed.