Thanks for reaching out to Q&A forum.
You can use any pdf package (example : PDFsharp) that would merge byte arrays to a single pdf file. There is nothing very specific to Azure Functions. Basically you can consider function like a light weight web app. Initially, I would suggest you to make sure the function runs in your local machine properly, once you have confirmed that it is working fine, then you can deploy to Azure Functions.
However there are some limitations for certain pdf packages in Functions, please review the below article