If you haven't tried it, make sure you are printing using the system dialog, and select "Open in Preview" in the drop-down list in the lower-left corner.
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.
It's an OS-wide feature. Any application can print to PDF without specific support from that app.
>However, Adobe has freely offered the specifications on PDF to other companies. Apple’s Mac OS X operating system has built in support for reading and saving of PDF files.
If you could render into a PDF context with your graphics engine (CoreGraphics) you would have a near print-ready document. (You still need to pass along additional info regarding whether to print duplex, number of copies, etc.)
So anyway, Save to PDF, Open in Preview were basically free.