Most users of Adobe’s hugely popular PDF Reader are content to use out-of-date and potentially insecure versions of the program, an analysis by antivirus company Avast Software has revealed.

Using numbers generated from the company’s large user base, the company found that only 40 percent of users had installed the patched Reader X version of the application, an important release that made its first appearance in November 2010.

A further 35 percent were using different increments of Reader 9, 14 percent Reader 8, six percent Reader 7, and 2 percent version 6, which dates back to July 2003. Versions 3, 4 and 5 took another 3 percent, which means the owners of those computers are running software to read PDFs that was released in the late 1990s.

“There is a basic assumption that people will automatically update or migrate to the newer version of any program,” said Avast CTO, Ondrej Vlcek, “At least with Adobe Reader, this assumption is wrong – and it’s exposing users to a wide range of potential threats,” he said.

The news that many Windows users allow old software to hang around on their PCs is not a huge revelation but the size of the problem is still a thorn in the side of companies such as Adobe, which was put under pressure to upgrade its approach to patching by security incidents in 2009 and 2010.

In May vulnerability management company Qualys published its own figures showing that 80 percent of browsers were running insecure Java plug-ins. Adobe’s Reader score was better but was still found to be out of date on 30 percent of browsers.

Looking for an explanation for the reluctance of users to update Reader, Avast quotes Adobe’s security head Brad Arkin.

“We find that most consumers don’t bother updating a free app such as Adobe Reader as PDF files can be viewed in the older version. In many cases, users only update when provisioning a new machine,” Arkin was quoted as saying.

“It is actually possible to be fully patched and up-to-date if you are running Adobe Reader 8 or 9. But I think a large percentage of users simply decline the update notification,” he added.

A large part of the problem is that on older versions updating was not automatic and required users to manually locate new versions, a recipe for trouble down the line. Some users also acquire Reader bundled on a new PC but never use it and for that reason probably see no reason to update it.

Qualys offers its free BrowserCheck tool that scans for out-of-date plug-ins direct from the browser itself.