Select the portion of the page you want to keep.
If you want to view the dimensions of the content you’re selecting, choose Tools > Show Inspector, then click the Crop Inspector button on your Mac, click the Show Markup Toolbar button on your Mac, open the file you want to change.
Choose Tools > Adjust Size, then select “Resample image”.
Enter a smaller value in the Resolution field.
The new size is shown at the bottom.
Tip: To reduce the file size of multiple images at the same time, Use the Touch Bar on Mac