Fujitsu ScanSnap N1800 Operator's Manual page 379

When [High Compression for Color PDF] is selected, characters and
backgrounds on the document are compressed separately. In order to do this,
the file size of the document that mostly consists of text characters becomes
smaller while keeping the quality of the characters as they are.
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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