对我们来说，合并pdf的***工具是什么?

And what's the best tool for us to merge the pdf's?

在Linux(以及Windows)上，您可以安装一个有用的小程序

On Linux (as well as on Windows), you can install an useful little program, pdftk. It works well to bind PDF's together. For example:

$ pdftk in1.pdf in2.pdf in3.pdf in4.pdf in5.pdf in6.pdf cat output out.pdf

其中in*.pdf是输入文件，而out.pdf是结果.在这之间，@ jerik已经给一个答案如何处理TOC.

where in*.pdf are the input files and out.pdf is the result. In between, @jerik already gave an answer how to deal with the TOC.