Why do you open the link from within a file document viewer? It will really hurt your performance since every deeplink will trigger the whole application to reload, so i strongly recommend against using deeplinks from within a mendix application.
I advice to find a way to open the form using a microflow (trigger). Maybe using a richtext viewer with microflow links is applicable to your case.
If you have no other option, I think a link with some javascript can do the trick:
<a href="javascript:void(0);" onclick="window.location='yoururl'">mylink</a>
Might do the trick.
Update
Are you able to specify the target of the link in the PDF? Then you should use '_blank' or '_top'