How to realize such requirement?

We are evaluating the REST APIs, and could not find a way to do the following requirement, can you please give me some suggestion for how to do that?

1. How to add a picture to the specified position of a PDF?
2. How to create PDF based on a .eml or .msg file?
3. How to retrieve attachments from a PDF?
4. How to add attachment to a PDF?
5. How to convert a non-PDF/A PDF to PDF/A format?
6. How to scale page content of PDF?

Scaling page content from the center of the page in percentage, after scaling, page sizing is unchanged, but there is more blank space around the content, page content is smaller, for example, the font size is smaller, image is smaller etc. Why we need this? It’s because we need to add data matrix code and company logo to those blank space.

I am looking forward to your feedback. Thanks!