Fujitsu ScanSnap N1800 Operator's Manual page 315

The color of an entire marked section should be even in thickness.
Good Example
Up to 10 sections can be marked in a page.
ATTENTION
In the following documents, highlighted character strings cannot be set as a keyword:
Catalogs, pamphlets, and other color documents
Documents including sections marked in two or more colors
Documents containing colors other than for marking (pens other than highlight pens are
used or the document is stained)
Documents containing a large number of marked sections in a page
When any of the following conditions apply, the highlighted character strings may not be able
to be set as a keyword:
The size of a marked section is out of the supported range
Marking lines are not drawn in an ordinary way (such as when a line is slanted)
Multiple marked sections are located close to each other
The color of the highlight pen is too pale or the color has faded
When applying markings on two or more character strings, the marked sections must be
separated from each other by at least 5 mm (0.2 in.). If marked sections are too close to each
other, multiple sections may be recognized as a single section.
In the following case, other characters close to marked sections may be set as a keyword:
When a marking is drawn over another line (upper or lower)
In the following cases, characters not highlighted by a marked section may be set as a
keyword:
Catalogs, pamphlets, and other color documents
Documents containing colored characters, figures, pictures, tables, lines, or other
elements
Documents containing marked areas other than those containing character strings to be
set as keywords
Tainted documents
Documents containing sections framed by a highlight pen
HINT
If setting a keyword is not successful, improving image scanning quality may help.
Keywords for PDF files can be checked from the Adobe Acrobat document properties menu.
Bad Example 1
(The characters are not
completely covered)
315
6.9 Setting the Scan Options
Bad Example 2
(The color is not even)