if your looking to convert pdfs, you could try this free tool:
http://cutepdf.com/download/CuteWriter.exe
1. Download and install CutePDF from the link given above
2. After the installation is finished: open the document
3. Click 'Print' button and choose a new printer called 'CutePDF Writer'
4. Click 'Printer settings', then 'Advanced' and then change page size
to 'Custom page size'
5. Now set the page size to 3,54 x 4,72 inch (or 90x120mm) and click OK
three times
6. Now click 'Print' and save the pdf file
I havent tried this myself yet, as I'm still umming and erring over which ereader to buy! :-)