Fujitsu ScanSnap S510 Operator's Manual page 208

3. In the "PDF file format option" window, select "Generate one PDF file per x page(s)"
and enter a number.
One PDF file is created every time when the specified number of pages have been scanned.
HINT
4. Click the [OK] button to close the window.
⇒
The setting is enabled from next scanning.
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In "Duplex" mode, two pages (front/back) are created per document sheet. It is
recommended that you clear "Allow automatic blank page removal"; select the
[Scanning] tab [Option] button to open the window. If this function is
enabled, there may be a discrepancy between the page numbers of the original
document and the resulting document in which blank pages may be deleted.