Since its release in mid 1993, Adobe Portable Document Format (PDF) has become a widely used standard for electronic document distribution worldwide in many institutional settings. Much of its popularity comes from its ability to faithfully encode both the text and the visual appearance of source documents, preserving their fonts, formatting, colors, and graphics. PDF files can be viewed, navigated, and printed with a free Adobe Acrobat Reader, available on all major computing platforms. PDF has many applications and is commonly used to publish government, public, and academic documents. Many of the electronic journals and other digital resources acquired by libraries are published in PDF format. As libraries grow more dependent on electronic resources, they need to consider how they can preserve these resources for the long term. Many libraries retain back runs of print journals that are over 100 years old, and which are still consulted by researchers. No digital technology has lasted nearly that long, and many data formats have already become obsolete and not easily readable in a much shorter time period. This document discusses ways that libraries can plan for the preservation of electronic journals and other digital resources in PDF format. After a brief discussion of the file specifications and the future plans for PDF, the article focuses on issues related to preservation of PDF files.