why?

cache headers are for files downloaded automatically by the web browser ie images, css, js files, not for files the user clicks to download. Shared files is not intended for linking to files from other content it is intended for users to download by clicking in which case cache headers are be irrelevant, the file should download when clicked.

Sorry but when a user clicks to download it should download.

Shared Files is for sharing files securely by roles. The shared files download checks the user roles and if allowed it binary writes the file to the response and it should always do that if the user is allowed otherwise it would be broken.

Maybe you should consider not using Shared Files and just upload and link to pdf files in the content editor, then you have standard links to the files and server configuration for static files can be configured for cache.

ie create the text for the link in the html content editor, select the text and click the link toolbar button in the editor, click browse server, upload your pdf from there and link to it. 

Hope that helps,

Joe

Note that I mean after the next release of mojoportal, unless you are upgrading by building your own package from the source code repository, in which case it is in there now.