can some one explain to be why the download button on elgg communiTy

Shows on mobile but the download button  on my elgg site dont show

sofar as i know there isnt a default download button in the core elgg. that is, it doesnt come with one by default, because most networks dont need it. however in my limited exposure i have seen two networks use them (elgg community like you said and team webgalli's website). im not sure if they have plugins for it though, theyre probably customised code.

try using a form like this:

<form method="POST" action="filename.pdf"> <input type="submit" value="Download!"> </form>

actually i think that will just open the file.
have a read of this:
http://stackoverflow.com/questions/3773055/how-to-implement-logic-for-my-download-button

@jsky: the download button shows when i visit the same page on my elgg site with computer and i download files.i can easily click downLoad button to get elgg plugins from this elgg community site while browsing with phone.