First the native Google Reader will only read Google source books, from what I have seen. You have a few options if you use a different reading application. First, I am assuming pfd=pdf, so you will need a pdf reader, which is not a great book reading experience, IMO and rtf (Rich Text Format) would be a poor experience as well. You can convert your files using Calibre to ePub or Mobi but the quality may not be acceptable. I suggest looking for your source in ePub or mobi and finding an app that supports your choice of formats.