PDF generation: Use system tools or make it optional

[elosha]

To make the script easier to use, it should adopt itself to available tools for generating PDFs.

• On MacOS try to use Preview.app or printing services
• In case nothing is available, don't worry and let the user keep the images
• Maybe the user wants to fix up the images before generating a PDF, so that could be optional or a seperate task

[elosha]

In case nothing is available, don't worry and let the user keep the images

Done.

Maybe the user wants to fix up the images before generating a PDF

For now, it can be handled by manipulating extracted files, re-running the script and using those existing files instead of extracting them again.