Open pdf problem

Server config : windows server 2003 , iis 6.0

When the client (use IE 7.0)?download a pdf file?some error occur :

If the pdf file size smaller than 500KB , it can open through the IE .

If the pdf file size larger than 500KB , it can not open through the IE . The IE load very very long time and then the screen became white.

However , if choose save the target , it can save the file in my PC and the file( larger than 500KB ) can open.

Another method to open the file , change the pdf preferences. Set do noth open the pdf through the IE.

What happen?? I don't know how to trouble shoot it.

thanks!

*p.s. Someone teach me use other pdf reader open the file through IE , it was success.

?What different between the pdf reader software.