Hello and welcome to Apple Support Communities muhfat,
Thank you for contacting us. You have come to the right place and we are happy to assist you. We understand you are unable to open pdf files. We can understand why you’d reach out for a solution. To start, make sure you have a current backup: How to back up your iPhone, iPad, and iPod touch & the most current operating system on your device. (Settings > General > Software Update) Are you able to open PDF files in Safari? Check out the articles below to open your PDF files.
Save and mark up PDFs on your iPhone, iPad, or iPod touch with the Books app
If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open
If you need further assistance after following the links above, please don't hesitate to reach out.
Thank you for contacting Apple Support Communities.
Best regards.