呵呵～问题解决了
这问题原来在国外早就发生过了，大概是他们应用的pdf比我们更早更广泛吧～国内只看见了一个帖子和这个有关。
下面是解决方法：
Solution
There are three different approaches to solve this problem if it occurs in documents originating from MS Office (or look-alikes):
1、Fix it in the RIP. On some RIPs you can change the separation algorithm to solve this problem. Contact your supplier to see whether your RIP has this functionality.
2、Fix it in the PDF. There are two Acrobat plug-ins I know off that are capable of replacing all RGB colours that contain equal parts of R, G and B by pure CMYK black. These plug-ins are Quite-a-box-of-tricks from Quite software and Pitstop 4.0 from Enfocus. Quite-a-box-of-tricks has the advantage that it can handled graylevels as well.
3、Windows 95 and 98 users can try to use the AdobePS 4.4.1 PostScript driver which has an option to output gray and black as CMYK colours. This driver can be downloaded from the Adobe website.
If the problem is caused by a colour management system, the solution is obvious: switch off the CMS.（英文少就不要翻译了吧，很容易理解。）
自问自答好有趣啊，呵呵！
