When creating PDF files, you can choose sampling to average and remove pixels from raster images, resulting in smaller file sizes. For example, if you’ve placed a 300-dpi image into a layout application but scaled it to 50% of the original size, the effective resolution of that image is now 600 dpi, far more than is needed when printing at 150-line screen. Sampling removes these “extra” pixels that are not needed, giving you a more compact PDF. There are three options you can choose from that controls how these pixels are removed: Downsampling, Subsampling, and Average Downsampling.
Downsampling will throw away that excess data. When creating PDFs, you can only downsample images; you cannot resample up to increase resolution. Resampling images in will analyze pixels in a given area and replace those pixels with their average value. Sampling options include Average Downsampling, Subsampling, and Bicubic Downsampling.
Average Downsampling. Average downsampling averages pixels within a sample area. The average pixel value is then assigned to the entire pixel area.
Subsampling. Subsampling simply chooses the center pixel value within a sample area. The center pixel value is then assigned to the entire sample area. Subsampling is the fastest of the three methods but yields the lowest quality and should only be used for things like charts.
Bicubic Downsampling. Bicubic down-sampling uses a weighted average within a given area to determine pixel value. With this type of downsampling, all pixels are compared and averaged to the center pixel. The new pixel is an average of these neighboring pixels. Bicubic downsampling takes the longest to process, but it yields the best results.
Resampling color or grayscale images requires an appropriate resolution setting (dpi) in the field adjacent to the sampling options pull-down menu. Typically, the value set in the dpi field should be twice the line screen (lpi) of the job printed. The field labeled For Images Above is for images that have a resolution above the assigned dpi threshold. Entering the same dpi value here as in the field just above it will further remove unneeded pixels from image files, reducing the size of the PDF file even further.