If you literally want this long description to be contained in a file, there are two ways to add a link.
The 1st way, a field with a file name is added to your database for each product, something like - $products[$i]['longdescription'] This can then be used to form the link.
The 2nd way, if you cannot or don't want to modify your database tables, would be to use the $products[$i]['catalog_number'] to form the file name in the link. This assumes that your catalog numbers can all be used as file names.
For the 1st way, the complete file name with a .txt or .html extension (or whatever you want) would be stored in the database. For the 2nd way, a .txt or .html would need to be added to the catalog number part to form the file name.
In ether case, the existing short description in the table would be changed. You can either change it entirely into a link or add a link to part of it.
To change the entire short description into a link -
To add something like "more info..." as a link onto the end of the short description -