After installing and running programs to find the lost password to the PDF file, follow these steps.
1. Specify the program path to the password-protected file.
2. Choose the type of characters that make up a password: letters, numbers, and so on. You can specify a different set of characters that make up the password. The more accurate will be specified character type, the quicker the password search, ceteris paribus. 
3. Enter the minimum and maximum length of the password and click "Add".
4. Specify the number of threads to find the password.
5. These are the minimum requirements for a successful search for the password to the PDF. At this point you can press the "Start", and then the program will start the password search process to PDF.
6. The program also allows you to plug in their dictionaries for password guessing.
7. If you have a list of prospective passwords to a file, you can specify it on the "Custom variants" tab that will significantly reduce the time to find the password to the PDF.
