Do the following steps:
1. Open a PDF in editing assistent of pdf24 PDF creator. (Print a document and click "Edit Document" or open the edit assistent and drag and drop a pdf into the right side of the window)
2. Click the right mouse button on the name of the document in the right side of the window.
3. In the opened context menu click Properties.
4. Choose the Security tab.
5. Enter an owner password and an user password. (The user password is the password which have to be entered if someone wants to open the pdf)
6. Click the OK button and than save the Document.