To send a PDF file using Facebook Messenger, make sure you have that PDF file stored in Google Drive or Google Docs. Open Google Drive or Google Docs on iPhone or Android and press the three dots ...

To transfer the PDF file, you must email it to your Send-to-Kindle email ID. This email ID is unique to your Amazon account solely to import documents into your Kindle library.

A reader would like to not only slim down his PDF files, but also e-mail them to his clients. With Automator it can be a one-step process.