Took me a bit of digging, but found just what I needed. I have been writing Tang Soo Do Martial Arts manuals for some time. I did the first three (Gup, Cho Dan, and E Dan on my Windows machine). Bought a Pi, now I do everything on my Pi. It is fast enough to do everything I need to do, but slow enough to keep me off the web (such a time suck).
Anyways, I found pdftk. It's an older program, but works just fine
sudo apt-get install pdftk
To merge pdf files, go to file folder with pdfs you want to merge:
pdftk *.pdf cat output nameoffile.pdf
That's it. Hope this helps.