Cant open/read PDF files. Pop up window asking to sign in

I think you mean OS X 10.13.6

I doubt that your OS makes a difference. I've been using Acrobat to read PDFs for many years, but only recently am I required to sign in.

Adobe's business model was changed some time ago. To assure a cash flow you are not able to buy any major app; you have to basically lease it, with payments every month. Perhaps they are collecting data to devise a pay to play for this app, too.

Personally, I'm going to start to look for alternatives.I am not in a business that I need their major apps 24-7-365. I designed, laid out and produced hard copy of a book using InDesign, but having to pay $30 every month just to have the app available was not worth it. When the project was over, I ended the contract.