Mac OS — decrease pdf size via Preview app

To decrease the size of the .pdf filled with images, in Preview you can go to Export and set Quartz Filter→Reduce File Size before saving.

If the images are too blurry for you, you can add your own profile.
1. Create a directory — /Library/Filters.
2. Add new filter file with unique filter — i.e. Reduce Size with good quality.qfilter.
3. The file should contain XML with new filter — you can base on the /System/Library/Filters/Reduce File Size.qfilter file or use the one below. Change compression setting, image size and add unique display name for your filter.

The problem is - the default filter used during conversion has very low conversion settings.

Thankfully, a custom filter can be added.

Adding custom filter step by step
Create new directory (if you don't have it) - /Library/Filters
Add there new filter file with unique filter - f.e. Reduce Size with good quality.qfilter
The file should contain XML with new filter - you can base on the /System/Library/Filters/Reduce File Size.qfilter file or use my below example. Change compression setting, image size and add unique display name for your filter.
Filter file structure/example
I marked key settings by comments.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Domains</key>
    <dict>
        <key>Applications</key>
        <true/>
        <key>Printing</key>
        <true/>
    </dict>
    <key>FilterData</key>
    <dict>
        <key>ColorSettings</key>
        <dict>
            <key>DocumentColorSettings</key>
            <dict>
                <key>CustomLHSCorrection</key>
                <array>
                    <integer>8</integer>
                    <integer>8</integer>
                    <integer>8</integer>
                </array>
            </dict>
            <key>ImageSettings</key>
            <dict>
                <key>Compression Quality</key>
    <!-- ====== Set your custom quality <0,1> ======= -->
                <real>0.75</real>
                <key>ImageCompression</key>
                <string>ImageJPEGCompress</string>
                <key>ImageScaleSettings</key>
                <dict>
                    <key>ImageScaleFactor</key>
    <!-- ====== Set your scale factor <0,1> ======= -->                 
                    <real>0.75</real>
                    <key>ImageScaleInterpolate</key>
                    <true/>
    <!-- ====== Set what sizes your images can reach ======= -->                    
                    <key>ImageSizeMax</key>
                    <integer>1684</integer>
                    <key>ImageSizeMin</key>
                    <integer>1200</integer>
                </dict>
            </dict>
        </dict>
    </dict>
    <key>FilterType</key>
    <integer>1</integer>
    <key>Name</key>
<!-- ====== Set unique display name for your filter ======= -->
    <string>Reduce Size Good Quality</string>
</dict>
</plist>

The problem is - the default filter used during conversion has very low conversion settings.

Thankfully, a custom filter can be added.

Adding custom filter step by step

Create new directory (if you don't have it) - /Library/Filters

Add there new filter file with unique filter - f.e. Reduce Size with good quality.qfilter

The file should contain XML with new filter - you can base on the /System/Library/Filters/Reduce File Size.qfilter file or use my below example. Change compression setting, image size and add unique display name for your filter.

Filter file structure/example

I marked key settings by comments.

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

<dict>

<key>Domains</key>

<dict>

<key>Applications</key>

<true/>

<key>Printing</key>

<true/>

</dict>

<key>FilterData</key>

<dict>

<key>ColorSettings</key>

<dict>

<key>DocumentColorSettings</key>

<dict>

<key>CustomLHSCorrection</key>

<array>

</array>

</dict>

<key>ImageSettings</key>

<dict>

<key>Compression Quality</key>

<key>ImageCompression</key>

<string>ImageJPEGCompress</string>

<key>ImageScaleSettings</key>

<dict>

<key>ImageScaleFactor</key>

<key>ImageScaleInterpolate</key>

<true/>

<key>ImageSizeMax</key>

<key>ImageSizeMin</key>

</dict>

<key>FilterType</key>

<string>Reduce Size Good Quality</string>

</dict>

</plist>

Further reading — https://apple.stackexchange.com/a/297418/347848

Mac OS — decrease pdf size via Preview app

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