display secured content message comes and thereafter, file download is not hapenning

There must be a link hardcoded to http:// in your page response - if you can get that removed and changed to a relative link by whichever component is responsible for it, then that would be best, however this may not be possible so your other options would be to use a STREAM profile doco here https://devcentral.f5.com/questions/can-f5-ltm-replace-http-with-https, or else enable SSL to the server hosting the element responsible for inserting the http:// - advertising/analytics often responsible for this type of thing.