psf Documents files?

What was the name of the application that created the documents? I can't tell from your post. The system may not be correctly recognizing the file format to begin with, which is why it is assigning an extension unfamiliar to you and also why you can't find anything to open it with. If you tell us the application that created the files originally and what type of files they are supposed to be... such as spreadsheet or word document or whatever we might be able to come up with a solution.

Yosemite is incapable of running Power-PC applications.

I don't know what would change the extensions. With Word X (and earlier versions of OS X), it was easy to create documents without an extension. But, again, I don't know what would have added the extension.

Because they were trying to figure out what software uses/creates a .psf file. So far the only reference to that format has nothing whatsoever to do with a text document.

If changing the extension doesn't work, you may have to write off the files entirely or find a PowerPC computer that is still running the application, open them, and save them in another format such as plain text then copy the files to a USB flash drive.

If you think they are Word docs, change the extension to .doc and see if you can open them.

Did you ever use a canon camera and its software? That is also a possibility.

What application did you use to create the PSF files?

Or, what kind of document is it supposed to be? Text, image, database, etc,?

A file with the PSF file extension is an Adobe Photoshop Proof Settings file.

The OS is incorrectly recognizing the files and adding a .psf extension by mistake. You need to change the extension to the correct extension. Odds are you will need to use .docx or .doc.