The Network is the Format: PDF and the Long-term Use of Digital Content

Abstract

The Association for Library Collections and Technical Services (ALCTS) defines the goal of digital preservation as “the accurate rendering of authenticated content over time.” To that end, the preservation community has delineated a set of possible approaches (migration, emulation, digital archaeology) to ensure fidelity in rendering, and has developed converging lists of recommended formats for different uses (text, still image, sound, moving image, etc.).Featuring prominently in many such lists is the PDF family of formats. Although it is a commercially developed format, it is widely seen as meeting many of the requirements deemed critical to reducing risk to the long-term viability of a format: it is in wide-spread use; there are many implementations, some of them open-source, of viewer applications; there is a publicly available specification of the format, control of which has been ceded to a public standards body (ISO). Further, there is ongoing work, under the ISO umbrella, to develop an “archiving” profile of PDF (ISO-190005), intended “to define a file format based on PDF, known as PDF/A, which provides a mechanism for representing electronic documents in a manner that preserves their static visual appearance over time, independent of the tools and systems used for creating, storing or rendering the files.”It is the thesis of this paper that these attributes of PDF, the admirable move by Adobe to place the specification of the format under non-commercial control, and even the developing of specifications for an archiving profile of the format, while necessary, are an insufficient warrant for the long-term usability of PDF instances. Further, the reasons why this is so clarify our understanding of what is required fully to characterize a file format. These reasons suggest that the current direction of the development of the PDF/A archiving profile may, ironically, constitute a significant departure from the warranties implicit in terming PDF/A an archival format. These reasons suggest that we not only must characterize a format instance, we must also characterize format renderers. They suggest that such anatomies of the “rendition stack” are important, not just for those who contemplate a strategy of emulation for re-enacting the original experience of an object's rendition, but also for the as-yet unanticipated uses, beyond “fidelity in rendering”, of the digital objects we preserve. They put in relief the challenges of encapsulation, or even multiple possible encapsulations, of a sufficient sub-graph of the network of information about a digital object, for effective future use.