Polar supports a feature to allow allow any users to add a document to Polar simply by clicking on a link / button.
The interesting part is that it supports any PDF document on the web that's publicly accessible.
Here's a button that links to the Bitcoin Whitepaper.
Here's what it looks like when rendered in the user's browser.
This allows for easy integration with Polar and 3rd party sites.
To create a link all you need to do is take an ordinary URL and prefix it with the following:
https://app.getpolarized.io/add
For example if you have a URL like:
https://bitcoin.org/bitcoin.pdf
Then you can create the Polar add document URL like:
https://app.getpolarized.io/add/https://bitcoin.org/bitcoin.pdf