PDF Transparency Guidelines

Be very careful when you use transparency (including drop shadows) when using page layout software. Strange things have been known to happen to transparent objects once a page has been exported, trapped and optimized, so check your PDFs carefully!

Also, be sure that your colors are built correctly. If you are printing a CMYK job, any spot colors accidentally left in the job will play havoc with your transparencies when they get converted to CMYK.