There are apps for pdf downloads. Meanwhile document assembly , or document generation can be very different things.
If by adaptive text, you mean variables or dynamic text e.g inserting the current customers name, inside a pdf template you might want to look for pdf services that also have shopify integrations. Doubly so if the customer is expect to put a signature on the pdf.
Automation apps like mechanic can generate and either add the file to shopify to the files admin or add it to a customer metafield, or email the customer the pdf.
https://tasks.mechanic.dev/demonstration-generate-a-file-and-upload-to-shopify
https://learn.mechanic.dev/core/actions/file-generators/pdf
(they now use the pdfcrowd.com api to generate the pdf from html)
PDF Theme customizations
Though if the PDF is the same for ever customer , a static file, that’s just file download handle with a bit of liquid and html calling the CDN url of the file in your files admin.
>download PDF
Though you want to change the download value so the file name probably to include the current date, end date, customer name, and company name, or some such.
If the file is dynamic such as inserting the customers name , that can be done on the frontend with theme-customizations using a javascript library that generates pdfs in the browser using html or images.
Going a bit further to bookend the a frontend generation use an upload-app, or form-app, or metafields-form app like helium-devs custmr app, or mechanic app, to then upload that generated pdf into the customer metafields in to be findable on that customer in the admin by staff.
If you need that type of PDF generation with automation apps or a frontend PDF theme customization contact me directly by mail for services.
Please always provide context, examples: store url, theme name, post url(s) , or any further detail.
Contact Info in signature.