Welcome to the Airtable community!
It sounds like you are an experienced developer. As you’ve noticed, the Airtable scripting environment has many limitations.
In general, any output that cannot be displayed as text needs to be sent to a 3rd party service. Fortunately, you can connect to a wide variety of API from an Airtable script. The trick is finding an API that will accept the input you have, generate the output you desire, and work with the limitations required by Airtable, all for a price point you can afford.
For example, you could look for a service that will accept your stream and and return a url for a file. Then you can display a markdown link to the file in scripting app, or upload the file to an attachment field.
You can also look into creating a custom app. Custom apps have fewer limitations than scripting, although there still are significant limitations.
The problem with most services that create PDF files for Airtable (FormStack, Documint, etc) is that you must create the template within the third party service; you cannot start with an existing pdf file. A workaround is to convert the editable PDFs into images and create a new document with the images as backgrounds and layer text on top of the images. The pdf will not be “filled in” but it will have the appearance of being filled in to a human user.
