What if you kill any active Adobe Reader process first, then as soon as it is closed, launch that inline function (or action to open the .pdf with Adobe)? (edit: forget this, edited and striked out - I re-read some of the posts here, and similar issues on stack overflow, I think I understand what might be happening...)
As a path format test, you should copy the .pdf file to the following literal path, and then try both the VoiceAttack 'Run an Application' action as well as Pfeil's inline function test above.
(test path - create this folder, copy pdf to it)
D:\test_folder\KTO_AIP.pdf
(and as a control, if you are so inclined)
D:\test folder\KTO_AIP.pdf
note the presence of a space in the path in the control, and use of underscore in the test path
This should uncover any formatting issues with the Adobe program and the path to the file to be opened, or rule them out as a likely potential culprit (which it is imho presently).