When you double-click a document, macOS will open that document in a default app. macOS uses the document’s file extension to figure out which app should open the file. As an example, by default, a PDF file called lighthouses.pdf opens in the Preview app because the Finder knows that everything with a .pdf extension should open in Preview.
But what if you would prefer to open .pdf files in Adobe Reader, or you want comma-separated value (.csv) text files to open in Numbers by default? To change any mapping or what app a document will open in by default, select a file of the type in question, so to change what PDFs open in, select a PDF, any PDF. Then choose File > Get Info to open the Info window. In the Open With section, click the pop-up menu to choose the desired app that you want that type of document to open in and then click the Change All button. So in my example, you’d select Adobe Reader and then click on the Change All button if you want all your PDFs to open in Reader instead of Preview.