Fix for split-pdf

[dotemacs]

Description of your pull request

Previously with-open was used to open the PDF document that would be split. Which is fine if you're just thinking about closing the file after you've split it. But it doesn't seem very useful, because once you've split the file, you probably want to do something with it.

I'm reverting this change introduced via 2ed58248360788cbdd4a4b5dbc95eadaddd9c767 just to get some opinions in this issue: https://github.com/dotemacs/pdfboxing/issues/46

Pull request checklist

Before submitting the PR make sure the following things have been done (and denote this by checking the relevant checkboxes):

• [x] The code is consistent with Clojure style guide.
• [x] All code passes the linter (clj-kondo --lint src).
• [x] You've added tests (if possible) to cover your change(s).
• [x] All tests are passing.
• [x] The commits are consistent with the Git commit style guide.
• [x] You've updated the changelog (if adding/changing user-visible functionality).