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If you know exactly what you want to do with your content, the most useful StackOverflow answers will come in response to code that you have written that implements some of what you're looking for.
For PDF generation, check out TCPDF, especially its examples. Multiple examples show you how to make multi-page documents. Example number 9 embeds an image on the page, and number 51 uses an image as a page background. TCPDF is roughly equivalent to FPDF, with a few more features and ongoing development.
For the epub and mobi formats, you'll need completely different tools.
There is a format convertor project on Github, but I haven't used it. It leverages other tools, like the ones below:
I know of one project that is a pure-PHP solution for generating epub content. And phpMobi seems to be an option for .mobi files.