Don't miss that you can easily use Notepad ++ has a hex editor. In the Plugins menu under Plugins Admin , you can add 'HEX-Editor'. This will allow you to view the hexadecimal notation for a file - see it at the binary level. As discussed in the Tip of the Night for February 1, 2016, a hex editor can be used to identify unknown files. by reviewing the bytes at the beginning of a file.
When a PDF is opened in Notepad ++, and you select Plugins . . . HEX Editor . . . View in HEX you will see at the beginning, "25 50 44 46":
. . . as shown in the below ASCII table, this stands for "%PDF".