PDF

Reduce the size of a PDF file consisting of scanned images

Platforms: Linux, Unix, MacOS X, Windows
Requires: ImageMagick

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

PDFs from scans are a very common occurence these days. Depending on the purpose conversion is sometimes required. It helps to understand how PDFs store the raster data internally to make to choose best option for the task at hand.

An overview of the can be found at wikipedia In short the below examples produce either an embedded JPEG or TIFF.

Assuming you have a few images laying around that need to be converted to a PDF file.

convert -repage <format> -compress <algorithm> [-quality <quality in %>] INPUT.tif OUTPUT.pdf

Creating a A4 PDF 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 in size A4 using the the TIFF format for storage 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