Documint creates PDFs, but it doesn't let you merge PDFs together.
For that, you would need to turn to a PDF merging tool that offers a REST API.
Here a few PDF tools that you can easily automate with Airtable & Make, but there many many more to choose from on the Internet as well. That list is just the tip of the iceberg.
If Make doesn't natively support the tool, then you can use Make's HTTP Module to access the REST API.
There is a small learning curve with Make, which is why I created this basic navigation video to help.
Also, note that Airtable's attachment fields are complex array fields, so they can be challenging to work with. I give an overview of how to work with attachment field arrays on this episode of the BuiltOnAir podcast.
p.s. If you have a budget for your project and you’d like to hire an expert Airtable consultant https://ecosystem.airtable.com/consultants/scottworld to help you with any of this, please feel free to contact me through my website: Airtable consulting — ScottWorld