Merge all pdf files from one folder

I would use pdfunite. It is nice and simple. cd to your directory. Then use something like this:

pdfunite *.pdf all.pdf

(Before running, make sure you don't already have a file called all.pdf in that directory.)

Thanks to steeldriver who showed me that this is doing the job:

pdftk *.pdf cat output mergedfiles.pdf

You can use qpdf as pdftk is not available in ubuntu bionic:

qpdf --empty --pages *.pdf -- out.pdf