Pages are what cost us money, so pages are what we charge for. Each plan includes an allowance; beyond it you pay per page at a rate that falls as you grow.
Reading a page costs us real money every time. An unlimited plan would mean either pricing everyone for the heaviest user, or quietly throttling. We would rather show you the meter.
One side of one sheet. A double-sided form photographed twice is two pages. Pages we fail to read are not charged.
Per page, quoted from a 50-page sample so you see projected accuracy and cost before committing. Rates fall sharply with volume.
On Enterprise, documents stay in your bucket under your keys. See Security for exactly what does and does not leave your infrastructure.