There aren’t even a handful of ‘awesome’ PDF editors for the Mac. Apple’s Preview app can do a substantial amount with PDFs that people might not be aware of:
https://www.makeuseof.com/tag/essential-tips-tricks-preview-mac/
For reading and annotations, I’m a big fan of the freeware Skim app.
As an alternative that runs better old slower, older hardware? Having used a 2012 Mac mini until 18 months ago as a home machine, I’d easily give the nod to PDF Expert over PDF Pen.