Oftentimes it can be convenient to merge different PDF documents in order to get a single, continuous document that can be easily sent via mail for review or correction.
If one has just a few documents, this can be done directly through the Preview.app application on the Mac, but for more documents (or when we want to repeat the merge many times) a command-line application can be very convenient.
On Linux, or on the Mac, poplar is the kind of set of tools that makes the trick (you can install it with Homebrew on the Mac).
In particular, you will find that the package includes a program called
pdfunite. Its usage is straightforward:
pdfunite file_in_a.pdf file_in_b.pdf file_in_c.pdf fileout.pdf