列印PDF檔，file size為何會大了很多？

It is because the printer does not support printing pdf directly. The printer driver needs to convert the pdf file to a format that the printer knows such as PCL.

2009-02-11 00:28:07 補充：
The typical way to solve driver related problems:
1. Download the latest printer driver from www.hp.com.hk
2. Uninstall current printer driver.
3. Install the new printer driver