What broke?

• Downloader
• Processor
• [X ] Converter
• Other

Mandown version
mandown 1.12.1

Describe the bug
PDF conversion fails with higher chapter/file counts

Steps to reproduce
Any comic with, I'd say, more than 100? chapters, run with --convert pdf, and it'll download the comic, then fail to convert with "OSError: [Errno 24] Too many open files"