Sometimes you need to merge PDF files made by someone else in a single file together. Your Windows or Mac user fellows will do probably by using Adobe Acrobat or the like. On Linux you don't need the Adobe software (although you can have it if you like). You can do it by the command line -it's Linux after all, isn't it?
The first method is by using convert , directly from the ImageMagick toolkit. You probably already have this one installed on your favourite Linux distribution; if not, almost every known Linux distribution has a package for it. Even if you don't need to edit PDF files you should have it, it is a wonderful swiss army knife for command line image processing. The syntax for merging PDF files is simple:
convert file1.pdf file2.pdf file3.pdf out.pdf
(that is, the last file name is the output file name). This usually works quite correctly, but 1)it is slow 2)I found sometimes has issues with image resolution. So I looked for another solution, and I found pdftk. The name stands for "PDF ToolKit", and really it is. It is a free (open source under the GNU GPL), wonderful command line utility that with a bit of magic allows you to manage PDF files from the command line. It works on Linux, Windows and Mac. pdftk can merge PDF documents, split PDF pages into a new document, rotate PDF Pages or Documents, decrypt and encrypt, fill PDF forms, apply a background watermark or a foreground stamp, burst a PDF Document into single pages... whatever.
The syntax for merging with pdftk is almost as simple:
pdftk file1.pdf file2.pdf file3.pdf cat output out.pdf
You just need to add the magic "cat output" between the input pdfs and the output file name. In comparison with convert, it is truly fast and in my experience gives better results. And it may come handy for when I have to work with PDF files in other ways. Also, KDE users may find nice PDF Concat, a Kommander script that acts as a simple pdftk frontend to merge PDFs.