Unable to convert pdf file

[fidev]

I've been working with my hosting company to get Imagick working to convert PDF files to image (either JPG or PNG, doesn't really matter for my purposes). We installed the latest version of ImageMagick, then after some searching and not finding an up-to-date version of the imagick dll found an older version and got that installed.

The conversion works form the command line, but whenever I try to convert using PHP, I get the error
Fatal error: Uncaught exception 'ImagickException' with message 'Unable to read image'
Imagick->readimage('pdfs/temppdf_.p...')

Can anyone give some insight into what could be causing the problem? I'm able to convert from one image format to another using the same PHP file, but I can't seem to read in PDF files at all.

For some additional info, this is a Windows server running IIS.

[fidev]

More info...

We installed an updated static imagick dll (php_imagick_st-Q8.dll from valokuva.org) and we're now able to convert some pdf files to image. However, with some pdf files we get

CGI Error
The specified CGI application misbehaved by not returning a complete set of HTTP headers.

Has anyone else encountered this and know what the cause could be?