System software version 7.0 or later has PC Exchange in the Install options. System 7.5 version 7.5.3, which includes PC Exchange, is available for download from here:

http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English-North_American/Macintosh/System/Older_System/

You might be able to just mount all the disks and find whichever one has PC Exchange on it, then copy it over.

Here's a premade preference file with most of the popular PC extensions already mapped to Mac apps:

http://net68000.freeshell.org/pcexpref.hqx

And here's Fetch for System 7, an FTP client:

http://fetchsoftworks.com/fetch/download/Fetch_4.0.3.sit