Open the PDF in Adobe Acrobat Pro
Go to the Tools menu
Select Protect > More Options > Remove Security
Enter the current password when prompted
Click OK or Remove Security
Save the unlocked PDF (Save As recommended)
Open the PDF in Adobe Acrobat Reader (if you only need to stop prompting)
Save the PDF to your device
Re-save it as a new PDF after opening (you may need Acrobat Pro to truly remove security)
Use online “Remove PDF Password” tools
Upload the PDF
Enter the password (if required)
Download the unlocked PDF
Use command line (QPDF)
Run: `qpdf –password=YOUR_PASSWORD –decrypt input.pdf output.pdf`
Use command line (Ghostscript)
Run: `gs -sDEVICE=pdfwrite -dNOPAUSE -dBATCH -sOutputFile=output.pdf input.pdf`