can not insert straight line with Shift key

Acrobat Reader would add a straight line with holding the Shift key as the "Draw Line" icon shows.

When i press the Shift key, click my mouse and move it, Acrobat Reader do show a straight line on my screen. But when i click my mouse at the end, Acrobat Reader will not stop the line. In fact, Acrobat Reader behaves like "crashed" and would not respones to anything but mouse moving: It keeps waiting for an end of a line comments.

Thus, I can't do anything in Reader and have to kill it in Taskmgr.exe of Windows. 

Anyone know what's wrong with it?