First install pdftk (pdf toolkit). Pdftk runs from the command line which means you should unpack the software into a directory that you can access easily. I put the file into c: for testing purposes
along with two pdfs called 123.pdf and 456.pdf.
If you run pdftk without parameters you get a quick output that details the most important parameters. Lets take a look at the possible operations:
123.pdf 456.pdf output 123456.pdf
This tells pdftk to merge the two pdf files 123.pdf and 456.pdf into a new pdf called 123456.pdf. If you want to merge more than two pdf files at once you simply add them between pdftk and output. You could also use wildcards to merge many or all pdfs with
a certain name.
*.pdf output 123456.pdf
pages from one or more pdfs:
A=123.pdf B=456.pdf cat A1-2 B4-5 output 1245.pdf
123.pdf output 123fixed.pdf
A=123.pdf 456.pdf input_pw A=password cat output 123456.pdf
Pdftk offers several more possibilities which are worth a look if you want to do something that I did not describe on this website. A version for Linux and Windows is available.