How to save multiple pdfs as one pdf

How to save multiple pdfs as one pdf

https://youtube.com/watch?v=82G1WjUCY3s

https://youtube.com/watch?v=PpmMdqgd-yw

https://youtube.com/watch?v=Ur241afTBAM

https://youtube.com/watch?v=_EvN0d67-_0

https://youtube.com/watch?v=xC_ljSa3Zzo