While I do now prevent tabs from being in the song titles and filenames, it was allowed for a long time, so if that is an older song, and there is a tab in the name, that could be related to the problem you are seeing. If that's not it, I may need you to backup the library under Settings->Backup and Restore->Backup Library, share a link with mike@zubersoft.com and I can restore your library and try the export to see what is going on. That's the quickest way for me to get you an answer. You can also try toggling the "Export iOS compatible annotations" setting, as that has rather large ramifications in terms of how the export works. If that's enabled, an entirely new PDF is generated from rendered images, which is much slower and generates a larger file. If it's not enabled, MobileSheets annotations are just added as PDF annotations on top of the pages which is much faster.
Mike
Mike