12. The printer of claim 8 , wherein the first interpreter is Adobe PDF Print Engine (APPE) and the second interpreter is a Postscript interpreter, wherein step (a) includes detecting a version of the PDF by checking the document header without analyzing a content of the document; and wherein step (b) includes: (b1) if the version of the PDF document determined in step (a) is one of a pre-defined plurality of PDF versions, selecting only the APPE interpreter to be used to interpret the PDF document without analyzing the content of the document; and (b2) if the version of the PDF document determined in step (a) is not one of the pre-defined plurality of PDF versions, performing steps (b3) to (b5) for each page of the PDF document: (b3) determining whether the page contains transparency data; (b4) if the page contains transparency data, selecting the APPE interpreter to be used to interpret the page; and (b5) if the page contains no transparency data, selecting either the APPE interpreter or the second interpreter to be used to interpret the page.
8. A printer comprising a print engine, one or more processors, and a memory having programs stored therein for controlling the one or more processors, the program being configured to cause the one or more processors to execute a process for processing a PDF document for PDF direct printing, the process comprising: (a) analyzing characteristics of the PDF document; (b) based on the analysis of step (a) and a busy or error status of the first and second interpreters, automatically selecting the first interpreter or the second interpreter or both interpreters concurrently to be used to interpret the PDF document; and (c) based on the selection of step (b), interpreting the PDF document using the selected interpreter or interpreters.