Software Design: Generating Documents Of Different Formats and Different Layouts

I am currently prototyping a piece software which must be able to generate different types of documents in different file formats. The document could be a letter, or a receipt, for example, requested as a Word document, PDF, or both.

I am using a Word and PDF library which each have their own separate APIs. Each different type of document can have many different layouts and formats, including custom fonts, images, headers and footers.

I want the design to be as abstract as possible, and avoid exponential growth of concrete implementations for every new type of document, layout, and file format.

First, I think I should abstract the PDF and Word document APIs into some more generic API. I feel like the adapter pattern is most suitable. For example, I might have an interface called DocumentAdapter, with two concrete implementations called PdfDocmentAdapter and WordDocumentAdapter.

I then want to be able to build any type of document, in any format, with any layout. I've therefore started to design a DocumentBuilder class with two dependencies: a document adapter, and a document specification. The specification should define the layout of the document - and be file format agnostic.

The DocumentBuilder will have a concrete method called BuildDocument, which builds the document for the appropriate adapter and specification - then writes it to the filesystem.

The problem is that the APIs for building Word and PDF documents are so different I don't really know how to best solve this design problem.

I could have something like LetterDocumentInterface -> PdfLetterDocument, WordLetterDocument, StandardPdfLetterDocument, ShortPdfLetterDocument, StandardWordLetterDocument, ShortWordLetterDocument, but this would lead to a ridiculous number of concrete classes.

Any hints or experience from building similar designs would be immensely appreciated