Jakob Nielsen's Alertbox, June 10, 2001:

Avoid PDF for On-Screen Reading

Summary:
Forcing users to browse PDF files makes usability approximately 300% worse compared to HTML pages. Only use PDF for documents that users are likely to print. In those cases, following six basic guidelines will minimize usability problems.

PDF is great for distributing documents that need to be printed. But that is all it's good for. No matter how tempting it might be, you should never use PDF for content that you expect users to read online.

Forcing users to browse PDF documents makes your website's usability about 300% worse relative to HTML pages. This is my rough estimate, based on watching users perform similar tasks on a variety of sites that used either PDF or regular Web pages. Because I have not performed a detailed measurement study of PDF on its own, I can't calculate the precise usability degradation. However, whether the true number is 280% or 320%, one thing is certain: the number is big and reflects significant user suffering in terms of increased task time and more frequent failures.

Problems with PDF

PDF was designed to specify printable pages. PDF content is thus optimized for letter-sized sheets of paper, not for display in a browser window. I often see users getting lost in PDF because the print-oriented viewer gives them only a small peephole on a big, complicated layout and they can't scroll it in the simple, linear manner they are accustomed to on the Web. Instead, PDF files often use elaborate graphic layouts and split the content into separate units for each sheet of print. Although this is highly appropriate for printed documents, it causes severe usability problems online.

PDF pages lack navigation bars and other apparatus that might help users move within the information space and relate to the rest of the site. Because PDF documents can be very big, the inability to easily navigate them takes a toll on users. PDF documents also typically lack hypertext, again because they are designed with print in mind.

In a recent study of how journalists use the Web, we found that PDF files sometimes crashed the user's computer. This happened most often to journalists working from home on low-end computers (especially old Macs). The more fancy the company's press kit, the less likely it would get quoted.

Because PDF is not the standard Web page format, it dumps users into a non-standard user interface. Deviating from the norm hurts usability because, for example, scrolling works differently, as do certain commands, such as the one to make text larger (or smaller). Also, after finishing with a PDF document, users sometimes close the window instead of clicking the Back button, thus losing their navigation history. Although this behavior is not common, it is symptomatic of the problems caused when you present users with a non-standard Web page that both looks different and follows different rules.

Guidelines for Using PDF

You should use PDF only for documents that users need to download and print. Which documents fall in this category? Anything with five pages or more is a good candidate, since users don't want to read a lot of text on the screen.

Usually, it's best to break up a long, linear document into multiple hypertext pages so that you avoid having a fat report in the first place. Sometimes big documents are needed, however, either because you have a lot of content or prefer a linear presentation. Classic examples of this are textbooks and manuals.

Sometimes it even makes sense to create a single PDF document that unifies multiple Web pages into a single printable download. If users typically need all of the information in a single session, they might prefer to print it out.

When you make a PDF document available for download, follow these guidelines:

Create a gateway HTML page that summarizes the PDF file in sufficient detail, including page count and file size. This will let users decide whether it is worth downloading.
State clearly that the PDF file is for printing only. Present the same content on other Web pages in traditional formats, and provide links to them for readers who prefer to read online.
From any other part of the website, link only to the gateway page, not to the PDF document.
Never let your search engines index the PDF file. Instead, ensure that your gateway page is indexed. Even though PDF files typically contain secondary background material, I've often seen them come up high in search result listings (possibly because big files are likely to contain multiple occurrences of query terms). In any case, usability suffers when users are unceremoniously dumped into a PDF file, especially if it's one that merely mentions their query in passing on page 47.
Ensure that your PDF document format is at least one version behind the latest offering. As with any Internet software, many users are slow to upgrade when new formats ship. PDF version 5 was released recently, but I recommend sticking to version 3 until 2002 (at which time you can use version 4; version 5 should not be used until 2004). I got many complaints when I made my 214 usability guidelines for ecommerce available for download in PDF 4 format recently: even among Internet professionals, there are still many people who have Acrobat version 3 installed on their machine. I am now wiser and will be keep using PDF 3 for any printable reports I publish in 2001.
Format your printable documents for different sizes of paper. Some countries use 8.5x11, while others use A4. Make sure your document will fit both.

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