If you're in the market for a document scanner for a specific purpose, which may include portability, overhead scanning, or receipt scanning, then you'll be better off with a special-purpose scanner than a general all-purpose one.
For instance, the Brother DSmobile DS-940DW weighs just 1.5 pounds and comes with wireless connectivity and decent scan speeds, making it an automatic choice for anyone who travels often and is looking for a portable scanner small enough to fit in their backpack.
Similarly, if your primary use case is scanning receipts and invoices, a scanner like the Epson RapidReceipt RR-600W, with specialized software to segregate recurring data (such as monthly expenses and vendor names), should be your go-to choice.