PDFs are a fantastic format, preserving the structure of a document, regardless of the device/screen used to view it, especially if like me you like to use fonts that are not that common. Not a lot people know this but if the person viewing your Word/Pages/PowerPoint does not have the same fonts as you, your document can look very different, bad different.
So, when sharing documents that do not require collaboration (or contain rich media, like video/audio), use PDF, which has the added advantage of stopping the recipient from changing your work, or at least making it difficult to change.
But what do you do if you want to change a PDF? Well you have to bear in mind that you are working against the grain, trying to change a PDF, is essentially something the format was designed to make difficult, it can be done, but don't expect it to be as easy or as 'flexible' as the software that would have been used to create the document in the first place.
What can you do?
As a general rule it helps to treat the PDF much as you would a printed document that you try change in the bad old days where the only way to change a document was to to use glue and scissors. So you can trim it, rotate, chop and crop bits of the top/bottom etc. You can add blocks of colour and images over the top, and also add text boxes. All of this can be done on your Mac just sing the regular, default Preview Application.
the text, ie, deleting and changing words is tricky, not impossible, but tricky. Most of the time you're better off to just copy and paste the text somewhere else and recreate the document. But there are ways... But not with Preview, or Acrobat Reader. Sometimes you can open a simple PDF in Word and edit it there... but most of the time, to edit text in a PDF you will need a specific dedicated Application, like the Pro version of Acrobat Reader, called Adobe Acrobat Pro. I'm using Acrobat Pro X, then you can possibly edit the text (with limitations), possibly. This support post from Adobe explains how, but in a nutshell, IF the document is not protected, and IF you have the same font as the PDF uses installed on your Mac, it can work, here's what you do:
Open the PDF in Acrobat Pro
Go to Tools, open up the Content panel, choose Edit Document Text.
Unless you don't have the font, in which case a little more work is needed, right click, choose properties, and change the font to isn that is on your Mac... In my experience there is a quite a bit of luck involved, some PDFs allow you to edit the text no problem, others throw up all sorts of problems, so remember there are no guarantees, editing text in a PDF is not a simple thing to do, but with a little perseverance it is possible.