Take a look at the File Types section in the PDF manual. On Page 124 (Page 132 for Adobe Reader 
) it is described how to customize the context menus.
If you'd like to apply some changes that are more substantial, there are some interesting threads here:
[Tip: Organise and Speed-Up Context Menus)
[Crash, exit or high CPU when right-clicking certain files)
[Position of context menu item)
Greetings,
Jan