How to edit PDF files

Hello out there!?

I started a proofreading/copyediting business last October and my son, who is 30, convinced me to buy an iMac. However, he's never around to help me use it, and he wants me to learn to figure things out on my own. I get it, but I'm 68. While I can be somewhat productive, I don't always understand the terminology or how things work. In addition, I am a visual learner. Up until now, I have edited my clients' documents using either Google.docs or Word's Track Changes. But yesterday I was offered a project that requires me to edit from a PDF. I was told I would need to have Adobe Acrobat Reader and that I could use the Free program. I spent time with Apple Support earlier and they helped me to download it. I even printed the directions from Adobe, "How to edit PDF files" so I wouldn't have to rely on my memory.

But, when I open the PDF that was sent to me as an example or open other PDF's that have been downloaded and then try to follow the directions I printed, I'm still unable to make any changes.

Can you help me? I need to know how to do this or I won't get the project offered to me. Once I learned how to get into the documents with Google.docs or Word, I had no problem. I need to know what I am doing by end of day, tomorrow.

Please help. I tried contacting Adobe, but the only option I was given was to go to the Community Forum. When I hit Post