The physical properties of high-quality paper make certain helpful functions possible or easier for users, including the following.

Lightweight so usually easy to carry, hold, and position

Thin so easy to grasp

Flexible, thus convenient to position, bend, and fold

Reflective, able to be illuminated for a wide range of brightness and contrast

Markable by a variety of means in a simple and uniform manner

Allowing detailed high-resolution markings

Opaque and two-sided so efficiently legible on both sides

Unpowered so portable and "always-on"

Stable, self-conserving and maintenance-free for many years

Cheap and movable, so many can be used, e.g. spread out side by side

Simple, easily learned, and widely understood methods of use

Digital display technologies used by today’s digital libraries (DL's) to deliver document images—a rapidly evolving ecology of desktop, laptop, and handheld computers, plus eBook readers, tablet PCs, etc— offer often contrasting affordances:

automatically and rapidly rewritable

Interactive

Connected (e.g. wirelessly) to a network and so can deliver potentially unlimited data

Radiant/back-lit, and so legible in the dark, but often limited in range of brightness and contrast

Sensitive (to, e.g., touch, capacitance), and so markable

This catalogue is incomplete but long enough to suggest the multiplicity of ways in which information conveyed originally as ink-on-paper may, and may not, be better delivered by electronic means favored by DL's. One result is that, as some researchers report, “paper [remains at present] the medium of choice for reading, even when the most high-tech technologies are to hand.” They point to four principal reasons for this.

Paper allows “flexible [navigation] through documents”

Paper assists “cross-referencing” of several documents at one time

Paper invites annotation

Paper allows the “interweaving of reading and writing”

It is illuminating to bear these considerations in mind when identifying obstacles to the delivery of document images via DL's.

Of course, efforts are underway to commercialize electronic document displays offering even more of the affordances of paper including flexibility, low weight, low power, and low cost.