1. A print document conversion apparatus comprising: a plurality of software-based conversion units that perform a software-based conversion process for converting print document data described in a page description language into page image data having a bitmap image format, each of the plurality of software-based conversion units requesting a hardware-based image processing apparatus that executes specific image processing in the software-based conversion process to execute the specific image processing, and generating the page image data including a result of the image processing executed by the hardware-based image processing apparatus in response to the request; and a controller that activates an additional software-based conversion unit that performs the software-based conversion process when a state where at least one of the plurality of software-based conversion units waits for the hardware-based image processing apparatus to complete the image processing possibly occurs.