How do I create a download link knowing the uploaded file ID?
First, load the file using file_load(FILE_ID) and get URI like below:

$file = file_load(1);
$uri = $file-&gt;uri;


After that generate link using file_create_url();

$url = file_create_url($uri);


Get file name also: 

$filename  = $file-&gt;filename;


And finally, create a link using l().

l(t($filename), $url);


and output will be like &lt;a href="http://d7.local/sites/default/files/pictures/picture-6.png"&gt;picture-6.png&lt;/a&gt;