A client recently asked about getting PDF files small enough to email easily, but without losing image quality – is there a trick to it? This is my reply.
The main factor in the file size of a PDF is photographs. Images accounted for nearly two thirds of the file size for a three-page document I was asked to evaluate. That’s what makes a PDF big. The file had a large photo in the background of the front page and other pages had images too, though not as large. It’s always a balancing act between image quality and file size. The smaller the file size, the poorer the image quality. There comes a point where the image quality is so poor, it defeats the purpose of including it. In some cases, you need to change how the document is designed to get the file significantly smaller.
In general, here are a few things to consider when trying to keep PDF file size under control:
1. Number and size of images: This is the biggest factor in file size. Photos or other ‘raster’ images are stored pixel by pixel. Logically, more and larger images will result in a larger document file size. Big background images look great, but they make for big files. If the image has a solid background colour that matches the document’s background colour (e.g. white on white), crop it as tightly as possible to minimize the image’s size. Remember that an image size causes an exponential increase in file size, so even a small reduction in the size of the image can make a difference.
2. Image resolution: The fewer pixels that need to be stored, the smaller the file size. There are two benchmark resolutions to remember: web/monitor resolution is 72 pixels per inch (ppi), while print resolution is 300 ppi. Depending on how your PDF will be used (printed vs. viewed onliine), you can play with the resolution. Print quality isn’t going to be acceptable below 150-200 ppi. Make sure you also tell the PDF generator to downsize everything above the target resolution. The default is to downsize pictures that are only more than 50% higher resolution than the target. For example, “Downsample to  ppi images that are over  ppi.” should be changed so that both numbers are the same.
3. Image quality: While resolution refers to the number of pixels, this setting determines the amount of information stored about each pixel. The lower the quality, the more ‘noise’ or image distortion that will appear. The higher the quality, the bigger the file size. Start with the ‘medium’ setting and experiment by going up or down from there to suit your needs.
4. Colour model: Converting CMYK images (for commercial printing) to RGB (for monitors and desktop printing) reduces file size.
5. Compression technique: Stick with the default setting in Acrobat: Automatic(JPEG). That gives the smallest file.
6. Other objects: Vector-based design elements such as boxes, lines or other sahpes add very weight to the file, so they can be the best way to make a document visually appealing when conscious of the PDF file size.
7. Special effects: You may have no photos in your PDF, but if you have boxes or other objects with transparency, drop shadows or other effects applied to them, they are converted to raster images to maintain the effect. Then you have the image quality vs. file size balancing act to contend with.
8. Raster versus vector: When there is a choice, use a vector version of your logo or other image. Vector images take much less memory. That means no jpg, png, gif or bmp logo files. If your logo contains both vector and raster elements, separate the two elements, rather than putting them together in one large raster image. Here is information on the difference between raster and vector images if you don’t already know.
9. Page count: Perhaps it’s stating the obvious that a shorter document will be smaller, but it’s sometimes forgotten. If you can edit the content down to reduce pages, it will help.
By the way, compression settings are applied pretty much across the board for a document. You cannot apply one set of rules to one object and a different set to another. The only differentiation is for colour, grayscale and bitmap images. There are a few other settings that can have an impact on file size (including bookmarks, subsetting fonts, including thumbnails, etc.) but generally, if you stick with the Acrobat defaults, you already have them set where you need them.
Do you have other techniques for optimizing file size of your PDF? Leave a comment and join the conversation.