Reduce the size of a PDF file consisting of scanned images
It is wise to scan documents at the highest resolution possible as downsampling can be done at any point. The fastest way I found so far the tools of the ImageMagick suite or with GhostScript.
Downsampling to 150 dpi without changing the type of image, aka -compress is as below.
convert -density 150 INPUT.PDF OUTPUT.pdf
Downsampling a PDF with images scanned at a high resolution to 150dpi converting the internal image to JPEG at a ratio of 80%. Useful for sending by mail.
convert -density 150 -compression jpeg -quality 80 INPUT.pdf OUTPUT.pdf
Convert images to PDF documents
Assuming you have a few images laying around that need to be converted to a PDF with a paper size in DIN A4.
convert -repage <format> -compress <algorithm> [-quality <quality in %>] INPUT.tif OUTPUT.pdf
The below example is with lossy JPEG compression at a compression ration of 80%. A higher number under quality yields a clearer image but requires more disk space. Note: the -quality option is optional but if you want to retain full control over the outcome I would suggest you use it.
convert -repage a4 -compress jpeg -quality 80 INPUT01.tif INPUT02.tif INPUT03.tif OUTPUT.pdf
For lossless PDFs using the the TIFF format for stoage there are two options either LZW compression or ZIP. ZIP seems to be a bit more efficient. Note the -quality field is not required.
convert -repage a4 -compress lzw INPUT01.tif INPUT02.tif INPUT03.tif OUTPUT.pdf
or
convert -repage a4 -compress zip INPUT01.tif INPUT02.tif INPUT03.tif OUTPUT.pdf