Reducing the file size of a .pdf

Using Ghostscript: https://askubuntu.com/questions/113544/how-can-i-reduce-the-file-size-of-a-scanned-pdf-file/256449#256449

Merging a series of pdf's

Command line, provided that pdfunite is installed

pdfunite in-1.pdf in-2.pdf in-n.pdf out.pdf

GUI

https://pdfsam.org/