[geekmaster]

The K5 has a LOT less free memory than previous kindle models (even those that have less total memory to begin with). Much more of its memory is used up by the built-in software. Early on when it was first released, several developers (including myself) bricked their kindles just by doing rather simple "ordinary" commands at the command prompt. Even now, developers routinely debrick their kindles. A significant amount of the problem is caused by running out of resources when you have extra programs running along with the kindle framework.

So it is not at all surprising that a large or complex PDF file could cause the K5 to run out of free RAM needed to display it, especially when the kindle has been running a long time since the last restart.

EDIT: On earlier kindle models you could shutdown the framework before running a large custom app, such as a custom PDF reader. On the K5, the system reboots when it notices that the framework (cvm and Xorg) is not in memory. You can pause and resume the framework, but you cannot unload it from memory, as far as I know. Perhaps there is a yet-undiscovered (or unpublished) way to do it. That would let you view large complex PDF files using a third-party PDF viewer program, like you can do now on a K3 or DX.