Hi, I want to display in a form (not Tempo, but Portal) a link to a documen

In either platform interface, this practice is neither recommended, nor easily implemented.
As an alternative, it is recommended that you analyze the business need to continue storing those documents out in a shared directory, rather than in Appian document management.

My requirement is: I have a shared directory with documents inside. In a Portal form I want to display a link to a document which is inside this directory. The user should click the link and download the document. Thanks!

This isn't a limitation of Appian per se. Microsoft and other browser vendors have gone to great lengths to ensure browsers are unable to access the local file system or network resources due to the security risks involved.

The normal structure for such a link would be something like file://<hostname>/<path> but I doubt a!safeLink will let you use it...