Welcome BookWyrm217. I'm old and so is my Sony - PRS-500. However, as I understand the 505 & the 700 can read epub format (the 500 doesn't).
That said, you will definitely be able to read LRF, the Sony format without DRM. And you should be able to read RTF (Rich Text Format). RTF can be ouput from MS Word or Open Office as well as many other word processors. This is good for your own original work. The main problem with RTF on the Sonys is they have not implemented it fully. You won't get TOC or images in RTF. Although you can convert ebooks using Calibre or Book Designer (both free downloads here on MR) to convert to LRF.
Places I know you can purchase LRF ebooks, are
Baen (SF/F but none have DRM) and
Fictionwise (their multiformat also do not have DRM & you can redownload in other formats). Probably
Books on Board also has LRF ebooks but I'm not very familiar with them.
O would stay away from PDF and Adobe for the 6 inch screens. PDFs can be particularly troublesome since they are usually made to be printed on 8.5x11 paper and appear too small on most readers (the Irex at 9 inch is one exception).
With the right software, and where it is legal to do so, you can remove the DRM from ereader, Mobipocket, & Micosoft's .LIT ebooks. You'll have to search elsewhere for these SWs since MR doesn not support it.
You can PM me if you have more questions.