The scanadf command apparently does not pass the --adf option to the
HP driver, so the device scans the platen instead of using the ADF:

===8<----------------------------------------
$ scanadf -o book.pdf -d 'hpaio:/net/hp_officejet_pro_8610?ip=192.168.178.99&queue=false'
Scanned document book.pdf
scanadf: sane_start: Invalid argument
Invalid argument
Scanned 1 pages
===8<----------------------------------------

Note that the “Invalid argument” errors do not stop the scanning
process from happening. But it scans just one page from the platen
instead of using the ADF. This suggests that the front-end may be
supplying the wrong option to the back-end to use the ADF (yet the
scanner carries on).

Some folks may be suspicious of a *.pdf filename when scanadf only
supports PNM formats. But I believe the HP driver is limited to PDF
docs when using the ADF. Nonetheless, “-o book.pbm” was also tried and
same result.

So naturally a test was done to pass the “--adf” device option to the
device:

===8<----------------------------------------
$ scanadf -o book.pdf -d 'hpaio:/net/hp_officejet_pro_8610?ip=192.168.178.99&queue=false' --adf
scanadf: unrecognized option '--adf'
===8<----------------------------------------