This part is to put that signature and dog pictures into a PDF that then gets emailed to [them]. So no, it doesn't have to be PDF but I cannot think of a format that would work quite as well.

TBF, it sounds like a good use of PDF and I might go that way too. The obvious alternative is HTML email or perhaps even better email them a link with a unique code to a dynamic page on your site which produces the HTML. They can easily then print that off (or to PDF!) if they want.

I have just started a new business partnership based around publishing so perhaps in future I will be doing more with dynamically generated PDFs.

If you are moving into publishing then I would be even more strong in my recommendation to look seriously at LaTeX. Its styles and classes allow for templating of documents or entire books as you see fit and with human-readable source there is the option of generating the entire thing programmatically.

More than half a lifetime ago* my classmates and I were given a course on LaTeX by our professor and while we could see the advantages he was espousing, the learning curve was indeed clearly steep. We continued to use various WYSIWYG alternatives (mostly DECWrite and/or Aster*x - I did say it was a long time ago) to produce our periodic assignments but eventually I and one or two others started to get to grips with LaTeX and produced some decent reports with it.

When the time came to write our dissertations the class was split pretty evenly between LaTeX and the other options. I was very glad that I chose LaTeX in the end. The fact that the source was plain text meant that I could work on it anywhere (Windows PCs, Linux PCs, Unix Workstations, the Vaxen - did I mention this was last millennium?) and it took up much less space so could be saved on even the smallest floppy.

One of my friends on the course persisted with using MS Word of all things. One day it crashed on her, corrupting the file and she lost not only that day's work but almost a week's worth to the last safe backup. I can still remember that event clearly to this day and it has always reinforced my decisions to go with open, text-based systems whenever possible and eschew proprietary binary alternatives. I have never regretted it.

OK, back to the topic. Go with LaTeX for all the reasons above. See some of the beautiful output it produces. Know that it builds on decades of work by Donald Knuth so you just know it's going to be stellar.

* Damn, I'm old.