Optimization and saving
When bandwidth is crucial, you can reduce size of PDF documents by applying optimization. Optimization removes unnecessary and unused elements from the PDF file and compresses images and forms to reduce the overall size of the document.
To optimize a PDF document, use the File > Save Optimized As… menu or press Ctrl+Alt+S.
The following dialog window appears:
Available optimization options are:
- Remove unused elements – this option checks the structure of the PDF document and removes any element that is not linked to or referenced by other elements in the file.
- Flatten form fields – this will turn form fields of the PDF document into same-looking fields that do not allow to enter data. Existing form values will be embedded to the document. This option is recommended if the document is to be printed anyway and hence interactive forms are of no use.
- Merge all layers – the optimized document still looks like the original but loses all information about layers. This option is only active if there are layers to merge in the original document.
- Color and Grayscale Images compression reduces the size of color and grayscale images in the PDF document by reducing their resolution (DPI) or applying one of compression methods (see below).
- Black and White Images compression reduces the size of BW images the same way.
Dots per Inch (DPI)
This option determines DPI of images. The lower DPI, the smaller is the size of compressed images. However, lower DPI leads to reduced quality as well.
Compare these two documents:
Original document (150 DPI)
Compressed document (76 DPI)
Master PDF Editor can compress images using several compression algorithms.
For color and grayscale images the following methods are available:
- ZIP is lossless compression that works best for images with large areas of solid colors. For photographic images provides low or no compression.
- JPEG is a lossy compression algorithm that can deliver a good compromise between image quality and size. This method yields the best results for images with smooth gradients (photographs). The JPEG Quality option allows you to set the desired compression rate where higher values mean lower compression and better quality.
NOTE: JPEG quality level of 75-90 provides the best size-to-quality ratio. Values of 60-70 offer acceptable quality and decent compression. Quality of 30-50 is used to achieve the smallest size of images where quality is not a priority.
Compare these documents optimized with different JPEG quality values:
JPEG quality 80
JPEG quality 60
JPEG quality 40
For black and white images the algorithms are:
- ZIP is lossless compression that is best for solid color areas or repeated patterns.
- CCITT Group 4 is the fastest lossless compression algorithm to reduce size of black and white images.
- JBIG2 provides lossless compression for BW images and delivers better compression ratio of images with text on them for the cost of compression speed.