This could go combined with these two links - 

First you create an Initialization method which gets triggered whenever you add a media file in Epi. As soon as you add it calls out the method to compress. The example - https://www.david-tec.com/2017/02/using-tinypng-to-automatically-compress-images-in-episerver/ is obviously doing it for Media files as Images. But you can call out an additional piece of code that shirnks PDFs to be specific. 

Just like this one - https://www.syncfusion.com/blogs/post/7-ways-to-compress-pdf-files.aspx#:~:text=Reducing%20PDF%20file%20size%20by%20optimizing%20fonts,document%20to%20make%20it%20smaller.

Give it a try

This is definitely great information. My client is fine with a paid service that exposes API to do compression. Wondering if there are any recomonded ones with nuget etc or I pick anything online.