PDF or portable document format files are a popular file type for documents because of their compact size and security features. Adobe has a couple of software products that they offer to open these types of files. One is the free Adobe Reader and the other is the not so free Adobe Acrobat which lets you create and edit PDF files.
One annoying feature of Adobe Acrobat and Reader is that they tend to open PDF files larger than 100% when you double click a PDF file. This may be ok for some people but for others it's too large and you end up having to reduce the page size down to a manageable level.
To change this setting, open Acrobat or Reader and go to the Edit menu and then click on Preferences. From there, click on the Page Display section and find the Zoom setting. Now you can change the default opening zoom setting for all PDF files you open with Acrobat. The default is Automatic but 100% usually works best for most people.