Fujitsu ScanSnap N1800 Operating Manual page 381

When the [High Compression for Color PDF] button is pressed, characters and
their backgrounds on a document are detected separately, and only the
backgrounds are compressed. For a text based document, the resulting file
size can be smaller while the characters remain clear.
However, note the following:
Photographs or pictures are recognized as background and compressed
at a high rate, causing degradation in the scanned image.
Thus, this mode is not appropriate for scanning photographs or other
graphical materials.
The contrast of the image may become higher.
If [High Compression for Color PDF] is selected, according to the type of
document, the file size may be larger than if a standard compression rate is
used.
Documents with many photographs or figures and only a few characters
Complex layout documents with multiple columns
Documents which have a background pattern behind characters
For documents with many characters scanned at 300 dpi resolution, a smaller
file size than that of High Compression for Color PDF is possible.
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