Fujitsu ScanSnap N1800 Operator's Manual page 322

HINT
When [High Compression for Color PDF] is selected, characters and backgrounds are
separated and only backgrounds are compressed. Therefore, the file size of a character
based document can be reduced without the quality of the characters being decreased.
However, note the following:
Photographs and graphics 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.
Compared to the standard compression rate, the file size may be larger when [High
Compression for Color PDF] is selected for certain types of documents.
Documents with many photographs or graphics and only a few characters
Documents with a complex layout such as 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.
6.9 Setting the Scan Options
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