Fujitsu ScanSnap S510 Operator's Manual page 196

Operation
4. Load the document onto the ScanSnap.
5. Press the [SCAN] button
⇒
Scanned documents will be output as highly compressed color images.
ATTENTION
176
.
In "Color high compression" mode, characters and backgrounds on the
document are compressed separately, and thus, the file size of the document
that mostly contains text characters becomes smaller while the quality of the
characters is kept high. However, this mode is not appropriate for scanning
photographs or figures because these graphical materials are recognized as
background and compressed as such, causing degradation in images. If this
setting is specified, the contrast of the image may become higher.