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How to reduce the size of scanned images in Preview

You can use Apple's Preview application to scan documents, but unless you adjust some settings the final file size may end up being very large.

Apple's Preview application initially was just a quick viewer for displaying image and PDF files in OS X, but over the years it has acquired a number of features that have turned it into a useful utility, especially since it is much simpler than tools like Photoshop and iPhoto while offering similar basic manipulation options. We recently covered how to use Preview for editing and organizing multipage PDFs, but in addition it can be used to import images from various sources, including locally attached or shared scanners that use Apple's Bonjour print services. Unfortunately, even when scanning relatively simple text documents, some people may run into problems in which the imported file ends up being exceptionally large, as MacFixIt reader "Emmy Zuckerman" describes:

I usually use Preview to make PDFs on my MacBook by scanning documents in and selecting "save as PDF" and the directory where I wish it to be stored. I usually select B&W and letter-size. The resulting document generally is very large in size, and often is too large to email to someone - especially if it is a multi-page document.

There are basically two types of graphics that PDF files will handle. The first is vector-based graphics, which are resolution-independent, and therefore can tell Preview or other PDF readers through coded directions how to draw image content so it will appear nicely on screen regardless of the zoom level. The second type of graphic is a "rasterized" or grid-based graphic that has a set pixel resolution. Unlike vector graphics, rasterized content has a static pixel content with a set resolution, meaning that it is not dynamically drawn but instead is just presented when embedded in a PDF file.

The scanner moves across the image in a manner according to your resolution settings, and imports the image in a rasterized format such as a BMP, JPEG, PNG, or TIFF file. After this is done, some file types such as BMP or TIFF will leave the imported data as is, and other types such as JPEG or PNG will compress the data.

Rasterized file types can be very large (especially uncompressed types like BMP and TIFF), which can result in large PDF files if they are embedded in them. This generally happens because people overlook the resolution and compression options when importing files with a scanner.

Changing the file format and resolution will have the greatest impact on the final file size of the imported image.
  1. Resolution
    To combat large file sizes when scanning, first consider whether you want the file to be printed or just viewed onscreen. For onscreen viewing, set the resolution to between 72 and 150 dots per inch (the Mac's screen is natively 72dpi, so don't set it below this or you will run into quality problems). If you want to print the file, then set it to between 150dpi and 300dpi. Only set it higher than 300dpi if you want to preserve detailed content or other high-quality features of the image in the scanned file (generally done for photographs or for archival purposes).

    Keep in mind that for an uncompressed file format doubling the image resolution will result in quadrupling the number of pixels in the image and can therefore drastically affect image size.

  2. Compression
    The second thing to do when trying to keep scanned file size down is to use a compressed file format when scanning, such as JPEG or PNG. For JPEG you can set the encoding quality to greatly cut down on file size. By default the quality is usually set to 80-90 percent, which is ideal for most situations. If you set it above 90 percent then the file size will start getting very large, very fast, and without much benefit to the naked eye. If you set the quality below that it will hurt the image quality of some photos. For most text documents you can further reduce file size by going down to as little as 20 percent without seeing much change in quality.

With those two options (resolution and compression) taken into account, you should be able to scan documents quickly and get them to be very small (between 10KB and 200KB for most text documents).

While Preview has the option to scan an image directly to PDF format, when you do this it will just embed a rasterized image into the document, and offer you few options to adjust the file size for the image. Therefore, instead of doing this, to keep file size low first import the file as an image file and then export it as a JPEG (you can use the quality slider when exporting to help reduce the file size). Then open the JPEG file and save it as a PDF to embed it in the PDF format. From here you can import it into a multipage PDF or otherwise manipulate it as a PDF document.

If you have previously scanned documents that you would like to reduce in size, open them with Preview and then save them as a JPEG file using the Quality setting to reduce the file size. Then check the resulting file by opening it in Preview, and save it again as a PDF. Now you have it in PDF form but containing the compressed version of the image, resulting in a smaller overall file size.

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