How can I shift page images in PDF files more to the left or to the right?

If you don’t want to write your own program code (as Nikolaus suggested), but use a Ghostscript commandline instead, you need to know 3 things:

1. PostScript has a setpagedevice operator that takes a PageOffset parameter;
2. Ghostscript will process snippets of PostScript code if you pass them with -c ... on the commandline;
3. Ghostscript can evaluate and apply (some) PostScript code even for direct PDF=>PDF conversions.

Now try this commandline to shift all page images by 1 inch (==72pt) to the left:

gswin32c.exe ^
  -sDEVICE=pdfwrite ^
  -o c:/path/to/output/pdf-shifted-by-1-inch-to-left.pdf ^
  -dPDFSETTINGS=/prepress ^
  -c "<</PageOffset [-72 0]>> setpagedevice" ^
  -f c:/path/to/input/pdf-original.pdf

_{(The -dPDFSETTINGS=/prepress I put in in order to not loose any picture quality of the scans…)}