How would I have it automatically upload posts?

I want to have users submit pdfs externally, and then the website automatically update with the posts having the pdfs embeded in them. I have no idea where to start with this.

Add a CMS layer like Forestry or Netlify CMS.

A user can upload an image normally like PNG but PDF could work too.

It gets added to assets directory.

Then they either make a post and add the link in manually.

Or you use a Ruby plugin for Jekyll that you write which creates a new post in the output directory for every PDF that exists.