Mac computers offer two ways of reducing the size of a PDF file without needing to install additional software. You can use the "Preview" image-viewing application to reduce the file size of a PDF with a one-size-fits-most filter. If you need greater control over the final quality than what the preset filter provides, you can use the ColorSync utility to create a custom filter to reduce the file size of a PDF and apply it in Preview.
Reducing a File's Size in Preview
Press the "Control" key and click on the PDF file in the Finder. Select "Open with" and highlight "Preview."
Click the "File" menu option and scroll down to "Save As." From the "Quartz Filter" menu, select "Reduce File Size.
Click the "Save" button to save the PDF in a smaller size.
Creating a Custom Filter
Click the "Go" menu option and scroll down to "Utilities." Double-click the "ColorSync Utility" icon.
Click the "Filters" tab in the "ColorSync Utility" window. Click the button next to "Reduce File Size" and select "Duplicate Filter." Click on the duplicate and enter a name for the filter. Click the button next to the duplicate filter. Click the buttons next to "Image Sampling" and "Image Compression."
Adjust the image sampling settings. A higher-scale percentage will give you better quality but will not reduce the file size by as much. You can also adjust the resolution and overall quality of the image sampling. The greater the resolution and quality, the less the file will be reduced.
Adjust the image compression settings. You have the option of leaving images uncompressed, setting the compression to the JPG format or allowing the filter to select the image compression mode automatically. The JPG format gives you the option of adjusting the quality using the slide bar.
Click the buttons next to "Image Sampling" and "Image Compression" to save the filter settings. Go back to Preview and follow the steps for "Reducing a PDF File's Size in Preview." Select the custom filter you created from the "Quartz Filter" menu.