Launch your version 8.5 copy of filemaker. Use Open from the File menu to open these files. Select the entire group of files to open in one batch. This will produce a converted copy with a .fp7 file extension. This file format is compatible with versions 7 through 11.
You will need to test these converted files in order to detect and make any small changes needed in order for them to work. In addition password access is significantly different than version 5.
I recommend reading this Knowledge Base article on converting to .fp7 format: http://help.filemaker.com/app/answers/detail/a_id/750/kw/converting
Having just converted some really old files, i.e. as old as FP3. If you have version 10, use it. Click all files of a solution to open and convert at one pass. Each will be converted to FP7. It will not consolidate the individual files to a single file of multiple tables. Work through the scripts to get the errors corrected.