You don't say whether you want to call it from a menu or a button or something else, but whatever the mode, you'll want to do if from a macro attached to that mode.

In your form, create a button (for example) and for the OnClick property, call the macro macOpenPDF (for example). In the Action column, use "runapp" and for the command line argument, you must reference the path and filename to Adobe Acrobat (the application) and then, the path to the actual document you want to open. In my test form, here is my command line argument for the runapp action:

C:\Program Files\Adobe\Reader 8.0\Reader\AcroRd32.exe c:\clc-drive.pdf

Save the macro and the form, then click that sucker and see it work! Of course, you can use this same method to open any document from any valid application, even text files in Notepad. But make sure your users will have the same applications at the same path destinations or the logic will fail.

Good luck- SLH