Ideally, you should try to keep a PDF document as small as possible even in the initial process of creating it. Here, a simple rule applies: less information means a smaller file size. Pictures especially should be compressed in advance. This is best done through picture editing programs like Photoshop or Gimp.
Besides the measurements and colour depth, the most important factor when doing this is the image resolution, which is indicated in units called “dpi,” or “dots per inch.” Halving the resolution from 600 dpi to 300 dpi noticeably reduces the size of a PDF, from approximately 4.55 megabytes to 2.68 megabytes. When choosing a resolution, you should always consider the purpose for which the PDF file is being created. If it only needs to be read on a computer screen, 72 to 96 dpi are sufficient. On the other hand, for a printing template or presentation, you shouldn’t go lower than 150 dpi, and 300 dpi would be optimal.
If you have an already-completed PDF in front of you, or you aren’t the creator of the file, you usually can’t avoid a belated compression if you prefer a smaller file size. However, this is commonly the most inconvenient method, because a belated reduction of the resolution is always made at the expense of image quality – even if the loss of image data is curtailed enough that a clear result is achieved. There are various tools for belated compression, which will be briefly introduced and explained in the following section.