Since Windows 8, PDF Files open in full screen mode. To stop PDF files opening in full scren mode in Windows 8, you need to download a program (such as Adobe Reader, which is a free PDF reader) to open PDF files.
Windows 8 opens PDF files in full screen mode by default because it uses a PDF reader app. Windows 8 has introduced a new type of program, called an app, which is basically a program that works on all Windows 8 devices (from phones to PCs) at the expense of opening in full screen only. Installing a program to open your PDF files with instead of opening them with the default Windows 8 app will stop PDF files opening in full screen.
If you don’t want to install a PDF reader program, then you can show more than just your full screen PDF using Windows Snap.