Programs like LibreOffice and PDF readers sometimes need to open a file (say, the user downloaded and opened a file from Firefox) as read-only. When that is the case, and the user wants to save the file for later, the user has the option to Save As... or Save a copy.
Why is Save a copy included if Save As... would do the exact same thing?
"Save as" saves the current file as a given filename. That may or may not be the first copy of the file. And in most applications, this path becomes the active file in the application.
"Save copy as" (et al) tend to create another file (which may or may not be the same as above) but won't change the active file (if it exists) and you'll still be editing the original copy.
TL;DR: "Save as" will change the current open document; "Save a copy", won't.
There should be three options (though not in all programs):
"Save" - This will save the document without asking for a new name or location. It will over-write the original.
"Save As" - This will prompt you to save the document using a dialog box. You will have the ability to change the file name and/or location. If you choose the same file name and location it will over-write the original. Your working document will be the one you just saved.
"Save A Copy" - This will prompt you to save a 'copy' using the same dialog box as 'save as'. You will have the ability to change the file name and/or location. If you choose the same file name and location it will over-write the original. If you changed the name or location of the document you will be working on the original document not the copy. This means that if you make additional changes and then hit save the original will be overwritten with the new changes, but the copy you saved earlier will be left at the state of the "Save A Copy".
