Hey, Neletra,
Thanks for stopping by the Apple Support Communities. From your post, I understand that when scrolling through a PDF file larger than about 150 pages, the document would skip all the way back to page 1. I'm happy to help!
First, check to see if this happens on multiple PDFs over 150 pages, and from multiple sources. If not, try deleting the offending PDF(s), force closing any open apps: How to force an app to close on your iPhone, iPad, or iPod touch - Apple Support, then restarting your iPad: Restart your iPhone, iPad, or iPod touch - Apple Support, and then redownload to try again.
If the issue continues after that, the next recommendation I can make is to reinstall the iOS software onto your iPad. This step should not remove any of your personal data, but I encourage you to first make a backup of your device: How to back up your iPhone, iPad, and iPod touch - Apple Support, then using the "Reinstall iOS" section of If you see the Connect to iTunes screen on your iPhone, iPad, or iPod touch - Apple Support to reinstall the iOS software (don't worry that the title of the article doesn't match your issue; the reinstallation steps are the same across the board and what we're looking for from it).
Enjoy your weekend!
