Browser Extension

Chrome and Edge

Install it at Chrome Web Store.

Firefox

Install it at Firefox Add-ons.

Safari Extension

Since I don't have an Apple developer account, I can't publish the extension to the App Store. If you want to update the safari extension, you need to manually re-install it.

Download Paperlib Safari Extension
Drag it to the Applications folder.
Open it, and then close it.
Open Safari - Preference - Extensions, Enable it.