pdflrf is great. Depending upon the content (and the status of DRM) you may want to consider calibre. RTF is not a good choice for documents on the Sony as neither the 500 or the 505 support graphics in RTF files.
Another option for you may be to create custom PDF files for the Sony. I have attached to this post a copy of the Sony provided guide for creating PDFs for the 500/505. (It has been posted elsewhere at MobileRead but I could not find it so I am attaching another copy rather than just promising to provide the link later.)
I have used the guide to create custom PDFs for a client for use by their technicians. The resultant documents were easy for them to use and very clear to read. They also had a number of diagrams in them and the PDFs were superior to the LRFs in this regard.
Many people do not like PDFs and they have gotten a bad reputation on the Sony 500/505. This is mainly from people expecting to read A4/letter sized PDF pages on a 6" screen with perfect results. When used with a PDF made for the 6" screen size (as
Feedbooks PDFs are for example, the results are wonderful.