First install pdfunite:
sudo apt update
sudo apt install poppler-utils
pdfunite source1.pdf source2.pdf merged_output.pdf
If a pdf file is located in a different folder, you have to add the path like this: $home/Downloads/source1.pdf
If you want to merge all pdf’s of the current folder you cant type:
pdfunite *.pdf merged_output.pdf
An alternative with GUI is PDF Arranger.