Well, if you’re willing to take stabs in the dark, things to try include:
When it’s open in the Adobe software, see if it lets you do a Save As… a copy might work if the original doesn’t. If it lets you print, you could try printing it to a driver that renders it as a .pdf and generate a brand new pdf that way.
Mess around with the original file in various ways. Try renaming the extension to .txt or .jpg instead of pdf and then copying that, and then renaming it to .pdf on the Mac if it copied. If that doesn’t work, if it’s as a txt file you might be able to open it in a word processor and look at the code, then copy and past the code into another text document, then save that – renaming it to a .pdf extension when you’re ready to go. I’ve also sometimes been able to loo at the inner PDF code and tear things out manually to get them to do what I want, usually in cases of corrupted files.
Sometimes the system level file settings are confused. You say the red only checkbox is unchecked. Well, check it, so it explicitly knows it’s locked, test it, then uncheck it to explicitly know to unlock it. It’s just forcing it to change the flag bit in case it’s confused.
Oh, and if the PC has an OS where you need to be an admin to do certain things, make sure you’re signed in on an admin account.
When you say the Mac doesn’t recognize it once it’s there, how do you mean that? You mean if you double click it it doesn’t open automatically in Adobe or anything else? That’s a common problem with files transferred from PCs anyway. You can try opening the application and then choosing File->Open and picking the path to the file once inside there, so it knows it’s supposed to be a PDF. It’s also possible the version of Acrobat reader, or the alernate progrm you are using (Preview, whatever) just can’t handle the PDF version, so make sure you have the most recent release of the Acrobat Reader (which they may have changed the name of if I recall correctly – just check on the ADobe site until you see the free app that reads the files).
I know some of this was suggested above, but, hey, stream of consciousness.