If a PDF file is encrypted, the user is required to enter a password before the full contents of the document can be viewed. This encryption was set using Adobe Reader, which is the official program of Adobe's PDF format. If you want to remove this password prompt and unencrypt the PD, you can do so using the native security tools of the same program.
Open your encrypted PDF file in Adobe Reader by double-clicking on its file icon.
Enter the PDF's password into the prompt and click "OK." This will unlock the full contents of the file.
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Click the "Document" drop-down menu, followed by the "Security" icon. From the list that appears, click "Show Security Properties."
Click the "Permissions" tab, followed by "Edit." Remove the password from the box on screen. Click "Apply." Your Adobe PDF file will now be completely unencrypted.