I put pdf files in weekly_pdf folder.. which is in the same directory as download_pdf.php file 
in that case setting the download folder to be site relative:
/weekly_pdf/
is not correct. instead it should be document relative:
weekly_pdf/
the file name is dynamic..
that was not clear from the original post.
in that case use the code i gave:
[php]<?php echo $_SESSION['MM_Username']; ?>.pdf[php]
for the File Name setting, and set the save as option to [FileName]
since a session is being used, you may need to add this code to line 1 to force the server to see the session:
php:
<?php @session_start(); ?>
if the download is still not working, go to the bindings panel and expand the Download File Binding group and add each of the bindings to the page to see information about the download that will help troubleshoot the problem.
