Well, first of all, Mac OS 8 is not free. Apple has never made it free (as they did with 7.5.2). So downloading it (legally) isn't going to happen any time soon.
If you want to buy Mac OS 8, you can get the install CD
here.
Besides, you'd be better off getting Mac OS 8.5/8.5.1/8.6 on that system as they have more PowerPC native code and would perform better.
If you have a PowerMac 7500, then you can network it with your PowerBook 2400c via the localtalk (printer) port on both systems. All you need is a printer cable and to enable AppleTalk on both system via the printer port.
As for working with files off a PC disk. Pre-Mac OS X systems don't generally recognize file name extensions. So if you have a file with .hqx on it, the system doesn't know what to do with it. You are going to need to manually open it in StuffIt Expander.
The thing about downloading with a Mac browser is that they are going to add resource info based on a helper application list. IE on a Mac would add to an hqx file a resource that tells the Finder that it is a StuffIt file.