I am using inbuilt application to open a pdf file on ios15 which allows you to make some changes and write on the pdf file but if I make some changes to password protected pdf file saved on my ipad ( like write an alphabet or rotate a page or delete a page ) and press done than I am not able open that password protected pdf file again using the same password and it shows invalid password.
Hello Rohit1828,
Welcome to Apple Support Communities!
We want to help you edit these documents without issue so you can get the most from your iPad.
Let's make sure you're using these steps to edit the PDF:
"Mark up a PDF
Open the PDF and tap 
to use the drawing and annotation tools (tap near the center of a page if you don’t see )."
This Apple Support page has the full details: Read PDF documents in Books on iPad - Apple Support
Or are you using a different application to view or edit the PDF?
Please also check for any pending iPadOS updates, which can help make sure the software is working as expected. This Apple Support page can help get those updates installed: Update your iPhone, iPad, or iPod touch - Apple Support
We hope this helps so you can edit and save the PDF without issue.
Cheers!
Issue with password protected pdf file
