There is a difference between loading a file and downloading a file. The following html code loads a file:
<a href="http://www.fbi.gov/top-secret.pdf">loading on the same tab</a>
After clicking on this link, your current tab will be replaced with the pdf-file that can then be downloaded. On right-clicking on this link, you can choose the menu item save link as for downloading the file directly. If you wish to obtain a save as dialog on clicking on such a link, you might want to take the following code:
<a href="http://www.fbi.gov/top-secret.pdf?download=1">save as...</a>
Your browser will download this file immediately if you choose to use a download directory in your options. Otherwise, your browser will be offering a save as-dialog.
You can also choose a button for downloading:
<button type="submit" onclick="window.open('http://www.fbi.gov/top-secret.pdf?download=1')">
save as...
</button>
If you wish to load the link on a new tab, you take
<a href="http://www.fbi.gov/top-secret.pdf" target="_blank">loading on a new tab</a>
A form element does not heed the directive ?download=1. It only heeds the directive target="_blank":
<form method="get" action="http://www.fbi.gov/top-secret.pdf" target="_blank">
<button type="submit">loading on a new tab</button>
</form>