[REVIEW ONLY] Open PDF document after export

This pull request implements #10.

@Liongold Please do not merge this pull request (yet). I’d like to get some feedback first and update this branch if needed.

@Denisov21: Mind reviewing this feature since you’ve originally requested it in #9?
Hm, I’m not too happy with adding this as a method to the `PDFDocument` module, but it shouldn’t prevent the pull request from being merged. Making a mental note to change it later…