Q: Is there a good way to make a PDF file non-editable?
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A: There are some options here:
1) If you want to disable interaction within your app you can prevent to user to interact with the PDF. For example, with the Android PDFNet SDK you can use PDFViewCtrl.setInteractionEnabled(false). This will avoid users to enter new annotations or editing forms.
Note that each application may or may not honor the security flags. Also, opening a flattened document in other viewers will not prevent them to add new content to the document.