1. you will need Acrobat standard or pro version (not the free one)
2. for easier computing, save all of your .pdf in the same directory.
3.select all the .pdf > right click on any of them > “Combline supported files in Acrobat…”
4. re-order the document by move it up/down then click “Combine File”
5. when it done, don’t forget to save the pdf to your hard-drive. that’s it.