To make this application work in the first place, avoid two things:

1. Don't move the default instalation.

2. The POSIX path to the PDF you want to convert must not contain special characters, including SPACES.

So, move/copy your PDF to the Desktop (which is a safe path: /Users/yourname/Desktop/) and remove any special characters and spaces from its name, using only basic Roman alphabetic characters, numbers, hyphen, dot and underscore.