If you are making your own documents, I suggest converting them to ePubs. PDFs are big and slow and the text doesn't reflow properly. Using ePubs gives you a lot more functionlity and reflowable text. There are several good, free programs to convert Word DOC and RTF to ePub.

If you do create PDFs for a PRS-900, I suggest that you make the document size appx. 3.5 x 6 inches and use small margins all around. I would keep the margins to about 0.25 inch or less. This size of PDF should look good on your 900 as it is the size of it's screen (excluding the information bar at the bottom of the screen). As mentioned above, avoid the Times New Roman font. A lot of people actually prefer a sans serif font on digital readers, but use whatever is easiest for your eyes to read.