PDF files can be protected in several ways: DRM (Digital Restrictions Management), user or author password. In addition, PDF can be a collection of images with text. User password is requested when trying to open a protected document. Without it, any actions with the file are impossible. It is difficult to reset or work around it. The most convenient way to remove your password is to use online services. There is no need to install anything on a computer, moreover, such an operation is not so often needed. Remove password from PDF, reset encryption and unlock protected document.
The official way to remove PDF password is to use Adobe's own tool Adobe Acrobat Pro. This is a paid tool with a 30-day free trial, so you can easily remove the protection inside the PDF viewer without the need for third-party tools. The official path from Adobe is good, but not very functional. It requires you to enter the user's password, even if the PDF file is not encrypted. To simplify the process and take advantage of additional features, it is better to opt for third-party utilities.