If you don't have an ADF for the machine you're feeding paper through, then you have a manual document feeder: yourself! We jest: but there are several devices on the market that are specifically called "manual document feeders"; they are all for specialized applications, of course.
Many ADF's have an extra slot or mechanism so that sheets can be inserted into the regular flow of documents: a familiar example is the provision of a slot in copiers for oversized sheets, which can thereby be fed in at the appropriate place - or a few such can be inserted without reloading the primary sheet feeder. Such devices use the regular supply feeders unless there is something in the auxiliary slot. Obviously a little pre-thinking is necessary if extra documents are to be inserted in the regular flow at specific places.
Kodak makes MDF's for its i600/780/800/1840/1860 series scanners. For the high-end models, these are flat-surface devices with adjustable guides, to allow for various-sized documents. The other models are vertical-drop-in slots, with (again) variable-width adjustments. The scanners in this series all have document extender devices, and run at from 60 to 154 pages per minute.
SEAC Banche, an Italian manufacturer, provides the SB 1500 scanning system with a motorized MDF. Not a contradiction in terms, this: it's a single sheet drop-in slot with a sensor which detects the paper and inserts it into the machinery very precisely. This seems to be a very comprehensive system, by the way: it does single-pass double-sided scanning, MICR and OCR reading, greyscale or colour, and variable resolution: it can also void document copies and print on them as well.
Canon, along with most copier makers, provides a device which is similar in concept to an MDF, but more so: an auxiliary or miscellaneous magazine, which can hold up to (typically) 10% as much as the primary feed; this is used to insert sheets in the regular flow like a "normal" MDF, but is principally meant to allow a variety of different-format documents instead of just one at a time to be inserted. The ease-of-use factor varies, as some work automatically but some need menus and choices to work (and some have not been well received!), but for the most part they work as one might expect.
The need for auxiliary magazines of this sort depends of course on the uses to which the machine will be put, but (at least so far as copiers are concerned) a one-sheet auxiliary slot is indispensable.
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