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Install pdftk - pdftk will be able to join or merge pdf's for you

Here is how to install

sudo apt-get install pdftk

Here is how to use pdftk after installing

pdftk *.pdf cat output single.pdf

this will merge all pdf's in the current directory and create single.pdf file

You can also merge pdf's by giving names individually

pdftk first.pdf second.pdf cat output final.pdf