Adjust JPEG quality

JPEG is a lossy format, which means that some image data is discarded when it is compressed, reducing the quality of the final file.

Original image; quality setting of 50; quality setting of 20

 Adjust the quality with the Quality slider pop-up menu in the Optimize panel.

A high percentage setting maintains image quality but compresses less, producing larger files.

A low percentage setting yields a small file but produces a lower-quality image.

 community help (adobe.com)