Continuing the discussion of InDesign transparency and PDF files from the last post: If you create pages that contain transparent objects, such as drop shadows, that interact with spot color objects in InDesign, you may get unexpected results.
Specifically, the transparent effects may disappear completely or be replaced by an opaque white box when you create a PDF file and view it in Adobe Acrobat or Adobe Reader. This will only happen if you create a "flattened" Acrobat 4 (PDF 1.3) file.
This is typically only an on-screen anomaly in the PDF file. InDesign sometimes must use overprinting to simulate transparent effects. To make the effect display correctly in Acrobat, turn on overprint preview. This is available as a preference in Adobe Reader 8 and all versions of Adobe Acrobat Standard and Professional. In Acrobat 8 overprint preview is located in Advanced > Print Production > Overprint Preview. When this is turned on, the transparent effects should display correctly.
Under normal circumstances, these effects should print correctly. One exception is if someone takes your PDF file and places it as a graphic in a Quark document. So always proof transparent effects closely.