Let's start with an existing PDF that has fillable form fields.
The end goal is to populate those fields with SFDC data and display the PDF in the browser.
What I've tried so far:
I've created an XFDF file that will populate the PDF. I can generate that file using apex. However, I'm not sure how to get from an XFDF file to a PDF file in the browser. It sounds like 3rd party tools or libraries are generally used to do that and I don't see an easy solution in SFDC.
Right now I'm thinking I'm going to have to recreate the existing PDF in VisualForce. I was hoping to avoid that but it doesn't sound like there is an easier solution out there.