Thank you Doron, the script is helpful.
To enhance it, I added a textbox field called "blank1" to each page of the PDF file. This field is solid white background to block out the main content of the file. By default, it's set visible.
If the file is determined by the script to be not expired, then the field is hidden, enabling the user to view the document. If JavaScript is disabled, the user gets a blank page. The field can be set to say something like "JavaScript must be enabled to view this document".
But will that work in online browsers ?? It will just show black pages in google drive preview ? Right ?