In printer driver, how can I make sure DrvLoadFontFile is called?

I have a printer driver that implements various Drv... functions, like DrvStartDoc, DrvBitBlt, etc. I want it to implement DrvLoadFontFile also, but that function never gets called, even when I print files from Adobe PDF Reader where I know have embedded fonts. What do I need to so DrvLoadFontFile gets called?

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