1. A print-document conversion apparatus comprising: a conversion-module activation section that is provided using a computer having a multi-central processing unit and that activates, as a process, a conversion module for performing conversion processing for converting print-document data, which is described in a page description language and which includes a plurality of units of processing, into print-image data in a bitmap image format or an intermediate-language format, the conversion module including a thread activation part that activates a plurality of conversion processing threads for converting the plurality of units of processing included in the print-document data into print-image data in the bitmap image format or the intermediate-language format, an assigning part that assigns the plurality of units of processing included in the print-document data to the plurality of conversion processing threads which have been activated by the thread activation part, and that causes the plurality of conversion processing threads to perform processing on the plurality of units of processing, and a shared cache that is shared by the plurality of conversion processing threads, wherein, when a conversion result that has been obtained by converting an object included in the plurality of units of processing, which have been assigned, into the bitmap image format or the intermediate-language format is stored in the shared cache, each of the plurality of conversion processing threads obtains the conversion result stored in the shared cache instead, of obtaining the conversion result by converting the object, and wherein each of the plurality of conversion processing threads, before converting the object included in the plurality of units of processing, inquires whether the object has previously been converted by the plurality of conversion processing threads and stored in the shared cache.