Using HTML::Entities, as the above Annymous Monk mentioned, would be a good bet. However, if that doesn't work for you, you can break up long lines of text in JavaScript like so:

var textmsg = "This is a long line of text that just " +
              "seems to keep on going and going and "  +
              "is just particularly large.";

<a href='some.site.com/address' onmouseover='popup(textmsg)' 
                                onmouseout='hidepopup()'> nameoffile.p
+df</a>
[download]

If the problem really is with the length of your line, breaking it up like so should fix that.

As a side note, are you sure you want to pop up a message whenever somebody puts their mouse over a link? Many might find that somewhat undesirable.. perhaps having the file description as text under the link, or even using image rollovers might work.

Browsers are beginning to offer functionality to block unrequested popup windows. I'm not sure whats considered "unrequested", but you may wish to test that out before your customers start complaining that they don't see any file descriptions :-)

Good luck!
-Eric

--
Lucy: "What happens if you practice the piano for 20 years and then end up not being rich and famous?"
Schroeder: "The joy is in the playing."