Well, first, as my own site is running 4.4.2, and the plugin works, it’s a little bit of a stretch to say that it “does not work in version 4.4.2.”
Next, a 500 error is an internal server error, not an error generated by the plugin itself. So, the key here is to find out why your server seems so stressed.
What web server are you running (Apache, IE, nginx, etc.)?
What version of the plugin are you using?
What other plugins do you have enabled?
Was WP Post to PDF working before you upgraded to 4.4.2, and if so, from which version did you upgrade?
How much memory is PHP configured to use (see WP_MEMORY_LIMIT)?
Are any PDFs rendering without 500 errors?
Is caching enabled for the failing PDFs?
Can you provide me with a link to the site to see this condition for myself?
I’m happy to help, but right now, I have nothing to give me any clues as to what the actual failure mode is. Again, a 500 error is a server error, not an error with the plugin. Now, whether the plugin itself is causing the server to report a 500 error, and the server is giving up on producing a PDF, we do not know. We do apparently know that when a PDF is requested, the server reports a 500 error, and that’s about it.