I think it was version 3.0 and later that incorporated ADE support when Sony transitioned to EPUB as their preferred format. When I used version 2.5, I think that I used to borrow PDFs from the library and transferred them to my 505 using ADE software. The latest version 3.3 is still available on the Sony US website for download. That's the version that I use with the 505 that I still have. What a great and reliable reader the 505 was!
Or maybe it's just best to wait until you decide what new reader to buy. Good luck!