Lindy reader is really good for reading comics in .cbr format. Just run WP7DesktopMarketplace and search for it on there, it downloads and runs everything apart from most of the Live games.
PDF are a different matter, the official PDF viewer just shows a black screen when viewing most pdf's. E.g. try some of the pdf magazines on this site. I think the best bet is to convert them to another format unless someone knows of a better PDF viewer.
Really recommend LindyReader as I have been viewing Archie comics with it, only downside is using dropbox to upload the comics then downloading them on your phone to its memory. If you have a low data allowance make sure it's plugged into the computer so it will use your home internet data allowance.