The retired Adobe Flash Player has nothing to do with PDF document viewing, as it was an alternative means that allowed Flash-specific website content viewing in supporting browsers. There is separate PDF viewing support in browsers.
The simplest means of viewing a PDF in macOS is to single-click the document, and tap the space bar to launch Quick Look. Apple ships the Preview PDF reader with macOS, as a more lightweight, and under-featured alternative to the free Adobe Acrobat Reader DC, which only Adobe can keep updated with the evolving PDF standards.
There are several single-purchase PDF editors available in the Mac App Store (PDFpenPro, PDF Expert, etc.) that cost far less than a single, annual subscription to Adobe Acrobat Pro, and unless you require the need to edit original PDF content, or add images, or an external signature image to PDF — you probably can get by with one of the reader chocices. You can change the default opening application by single-clicking a random PDF, and then pressing option+command+i to bring up the Get Info panel. On that panel is a Open with section, where you can set it to Preview, and then click through the Change All… button. Afterwards, PDF would default to opening in Preview.
Apropriately named PDF documents can be sorted with Apple's Finder, but if you need them indexed for searching, and Apple's Spotlight is insufficient for that requirement, then you may still require a PDF editor that supports that capability, and that might still be Adobe Acrobat Pro.
