The Quartz filter supplied with macOS to reduce the PDF size often leads to blurry files. But there is a trick.
If you export documents with placed images in an application such as Word or Pages as PDF, the PDF files often become very large and can no longer be sent by e-mail with a clear conscience. This often also applies to documents that you have scanned yourself. Often, however, this file size is not necessary at all, since the PDFs are only intended for information and to be viewed on the screen and therefore high resolutions are not required. With preview, such PDFs can be reduced to a more mail-friendly size. Apple has provided an option for PDF export or for saving as PDF in order to quickly reduce the file size. To do this, open the PDF in preview and select either “Export” or “Save As” in the “File” menu, select “PDF” as the file format and “Reduce pdf file size” in the “Quartz filter” drop-down menu. Then Preview recalculates the images and scans with a lower resolution during export. However, this usually leads to blurred PDFs, since with this filter the images and scans are evaporated to a certain number of pixels. The poor quality is particularly easy to see on retina displays.
You can remedy this with your own quartz filter. To do this, open the ColorSync utility, which can be found under “Applications> Utilities”, and click on “Filter” in the toolbar. Now click on the plus symbol in the lower left corner of the window to create a new filter and name it. Then click on the gear symbol to the right of the new filter and select “Add component for image effects> Image adjustment (color)” in the menu that appears. In the settings under Image adjustment, set “High” in the “Quality” drop-down menu, mark the checkbox for “Set resolution” and type in “150” as the resolution. Then the images are still sharp even on retina displays. Now right-click again on the gear symbol and select “Add component for image effects> Image compression” in the menu. In the settings, select “JPEG” for “Mode” and leave the slider for “Quality” in the middle setting. Now you exit the ColorSync utility, restart Preview and then you can select the new filter for the PDF export.