When you double-click a document, macOS uses the document’s file extension to figure out which app should open the file. So, by default, a PDF file called laser-squid.pdf opens in Preview 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? To change any mapping, select a file of the type in question and choose File > Get Info to open the Info window. In the Open With section, click the pop-up menu to choose the desired app and then click the Change All button.