Often when I fetch a datasheet or manual, the modification time will be now if the web server doesn't send the modification time metadata, or even if it does, the modification time of the file on the server, which is often later than the generation time of the PDF file. I like to see how old a document is from the file modification time. This information is in the PDF metadata, and hopefully hasn't been altered.

So I wrote a Python utility to do what I wanted, released at https://gitlab.com/retiredfeline/setpdftime It might be of use to you.