PDF supports a set of permissions that allow authors to request certain actions are disabled, such as editing, annotating, or printing documents. Due to the nature of the PDF format, these restrictions cannot be guaranteed. Some PDF readers enforce these restrictions and some do not. They are not enforced in PDF Viewer for iOS and Mac.
For documents you have authored, you can add an owner password and set these permissions directly in PDF Viewer before distributing your document. You can also edit permissions for existing documents, if you have the document password. Note that this functionality requires a PDF Viewer Pro subscribtion.
You can also edit PDF document permissions in third-party applications such as Adobe Acrobat.