Users can view PDFs but preventing them the download

**Is your feature request related to a problem? Please describe.**
Sometimes allowing users only to view some PDFs is preferable, preventing them from downloading them.
Sometimes you may want to share PDFs, but you want to prevent the download for various reasons.

**Describe the solution you'd like**
So, you can implement the plugin code with a feature that allows avoiding downloading PDFs.
In that way, PDFs can remain "embedded" in the webpage, and users can only view them page by page, but they should not be allowed to download PDFs. It's like a security mode.

I much appreciate it if you consider this suggestion soon.
See issue #26