how to open PDF documents

Adobe Flash is now dead and has absolutely nothing to do with PDF documents. You can open the PDF document with Apple's Preview application, by a single-click on the PDF, and pressing the spacebar (Quick Look), or by clicking on the PDF link on a website and viewing it in Safari, or other browsers that have built-in PDF support.

The Quick Look (spacebar) feature is a useful time-saver, but does not allow selection, copy, or paste from the viewed document (as it once did several years ago).