BTW, if you are trying to save a protected PDF from Adobe reader that won't let you save it in an unprotected format that's compatible with Preview, there is "print" document trick you can try since Adobe Reader blocks the system print dialog: pause the queue for your printer (if you don't have a printer, just "install" a network postscript printer), and print the document. The document is then saved to a non-secured temp PDF file in /var/spool/cups, to a filename starting with "d" (ls -l -t to find the most recent file). That file is normally deleted after being sent to the printer, but my pausing the print queue, the file is there long enough to copy somewhere. Rename the document, and you're good to go.

This is a trick from the Mac OS X Hints days, and still works, as of at least Catalina.