You may need to merge several pdf-files into one.

There are several tools. Some of them you have to pay for.

If you are in a linux environment, try this:

gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=final.pdf filea.pdf fileb.pdf

continue with filec.pdf etc.

Some say it is slow.

Others say, yes it is slow, but it produces smaller final files, and are able to handle “difficult pdfs”.

I say it is nice that it can be done from the command line.