Adventure Text

Are there any plans to include adventure text within Roll20 interface, ie the contents of a PDF, so that the GM does not have to refer to any other software when running the game? This would also make creating stand alone adventures for the marketplace easier.

Not entirely sure what you're asking, but, can you use the Handouts section for this?  You can already put whatever Text and Pictures you want under Handouts.  And you can make a Handout visible to certain players, or all players, or hidden to all but the GM. I haven't played an Adventure from the Marketplace yet, so, I may be ignorant if you are asking for a feature that is associated with those.

I've not seen any adventures from the Marketplace either. What I'm talking about is having the whole contents of the PDF, contained in R20. Handouts could work however there would need to be some sort of indexing system I think.

One of the Marketplace adventures comes with a PDF but you read it in the standard way (outside of Roll20). - Gauss

Ideally everything would be in R20, so less flicker between programmes.