Combining PDF files with Pdftk

A user manual I downloaded was split into about 20 pages, and I wanted it combined into a single document.

Pdftk did the job.

After looking at the official examples, and since all of my PDF's were numbered sequentially, I successfully combined them all using a wildcard:
pdftk *.pdf cat output combined.pdf
If they are not numbered sequentially, you'll have to include each filename:
pdftk 1.pdf 2.pdf 3.pdf cat output 123.pdf
If you prefer a GUI, install PDF Chain.