Greyscale documents and images are composed of gradients of black. Documents scanned this way can be saved as a PDF file of 200 to 600 dpi. If you are saving the document online, you can reduce the file size by using the GIF format, but your document would lose the text editing and search format of PDF (unless the downloader does a file conversion at their end). For online use, businesses usually choose a smaller resolution of 200 dpi.
Colour photographs are often scanned at higher resolutions to capture their detail – typically from 600 to 1,200 dpi, with the higher resolution used for the best archival images. However, for web use or to save hard disk space, 200 dpi is the standard choice of most businesses for most colour images and colour documents.
Colour photographs on their own should be saved as .png files while colour documents are best saved as PDFs, with their text searching capabilities.
