Well in such a case, it seems like you have three options:
Option 1:
>> Create a custom link using .cshtml file and provide a download link in the file by creating a method in the controller.
>> Try it with HTML5 Attribute i.e. Download="FileName.extension" ,it works well.(<a href="/files/adlafjlxjewfasd89asd8f.pdf" download="expenses.pdf">Download Your Expense Report</a>)
Option 2:
>> Try by creating a category. But its hard to identify the category without placing a hard code check for the same.
If you place a hard code check then replace the url of the category.
Option 3:
>> Create the category and add it to the top menu.
>> On the category page, insert a javascript script block with an HTML redirect to where the PDF is located.
If you might need more help with it, feel free to get in touch and I'll be happy to get you a consultation with our
nopCommerce developers.