1. A non-transitory recording medium storing a computer readable program that is executable by a computer to convert first page description language data described in a first page description language to second page description language data described in a second page description language, wherein the first page description language is convertible to the second page description language using a computer-executable general-purpose conversion process, the program being executable by the computer to cause the computer to perform functions comprising: a) analyzing the first page description language data described in the first page description language to determine whether a repetitive pattern image portion is included in the first page description language data; b) in a case in which it is determined that the repetitive pattern image portion is included in the first page description language data, separating the repetitive pattern image portion from the first page description language data; c) generating first partial data in the second page description language by converting the repetitive pattern image portion separated from the first page description language data to the second page description language using a unit image repetitive rendering command in the second page description language, without performing the general-purpose conversion processing on the repetitive pattern image portion, wherein the unit image repetitive rendering command is a rendering command to render the repetitive pattern image portion of the first page description language data by using a single image datum relating to a unit image which is repeated in the repetitive pattern image portion, wherein the single image datum relating to the unit image is described only once in the first partial data, irrespective of a number of repetitions of the unit image in the repetitive pattern image portion described in the first page description language data; d) generating second partial data in the second page description language by converting a portion other than the repetitive pattern image portion of the first page description language data to the second page description language using the general-purpose conversion process for converting the first page description language to the second page description language; and e) combining the first partial data and the second partial data to generate converted second page description language data that relates to the first page description language data, and that is described in the second page description language, wherein the general-purpose conversion process performs conversion such that, if the repetitive pattern image portion included in the first page description language data were converted to the second page description language using the general-purpose conversion process, the repetitive pattern image portion would be converted in expanded form in the second page description language, in which the image datum of the unit image is repeatedly described according to the number of repetitions of the unit image in the repetitive pattern image portion described in the first page description language data, whereby conversion of the repetitive pattern image portion using the unit image repetitive rendering command achieves a reduced data amount of the converted second page description language data as compared to a case in which the repetitive pattern image portion is converted using the general-purpose conversion process.