If you are using InDesign to create the PDFs, then an option would be to use HELIOS PDF-native OPI. OPI automatically creates low-res "layout" images that you place in the document. This should speed the transfer time and response time of InDesign. To produce the final PDF document, export with "omit images" (resulting in a very small document), and the OPI software on your server will restore the omitted images. Different versions can be produced with different image resolution and ICC color space settings. All other PDF properties (transparency, etc.) are preserved.
Tom Hallinan
HELIOS Software