Considering that pdfunite is part of poppler it has a higher chance to be installed, usage is also simpler than pdftk:
pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf
Just make sure you remember to provide out.pdf, or else it will overwrite the last file in your command
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
