I can't read old pc files on my Mac. They appear as "Unix" files. How can I read/convert them?

They don't need to be converted. While there are some actual Unix files to be found on the hard drive, when a file is labeled as such, that's OS X's version of Windows marking an unknown file type with a dog-eared page with the Windows logo in the middle. In other words, OS X is saying, "I have no idea what this file is."

Since everything in Windows has a file extension (OS X is mostly that way now, too), what are some of the ones OS X says it can't read?

Kurt , the other writer never replied to you but here I am two years later with the same query, and in my case the extensions that mac can't read are .ser or .brf or .rju just arbitrary extensions I gave files some years ago to identify for myself what they were but I didn't understand the significance of what I was doing and now since chaning to a Mac I cannot access these and really need to, want to. ..

Since the extensions aren't relevant to the app they came from, you're going to have to try and figure it out.

One way is to drop a file onto TextEdit (if it's not too big). A straight text file will be completely readable, so you know right away that's what it is, and it should have a .txt extension. If it's completely readable but has formatting such as bold, colored text and such, then it's a rich text document and should have a .rtf extension.

Then comes the fun part. Binary files. TextEdit will show binary data as a bunch of nonsense characters, but there is almost always clear text in the file. Most commonly (but not always), the app which created the file will have such text towards the top or bottom that tells you (as an example) it's an Excel or Word document.

It's not at all uncommon to find clear text throughout a binary file, you're just more likely to find info of the app that created it towards the top or bottom. From there, you can change the file name to the correct extension.