how to add a PDF-file (datasheet) in the product description field

I have been using Open Cart since 2012 and I am very happy with it.

My OC version is 3.0.2.0
Theme: standard
My website (product page involved): http://www.amelectric-shop.eu/index.php ... duct_id=82

Since beginning of this year, we have migrated to the latest version 3.0.2.0. In the old version 1.56 we could easily attach PDF-files to a product-file. We just put in a PDF-picto and attached the URL of the correct PDF-file. In the old version there were two URL buttons, one to add a URL to a word or picture, and the other to remove the URL. When I used the “create URL” button I got a menu with a "browse server" button. With this I could easily select the correct PDF-file.

In the latest version 3.0.2.0 this option seems to work differently, and I have not been able to find out how I can do this.
There is only one "Insert Link" button. In the Insert Link menu I don’t get the button "browse server" anymore to select the folder and file. I need to enter the correct URL in the URL field. When I attach a URL to the word "Datasheet" and I click on the it in the front end, I get an error message that tells me I have no rights to open this page.

What am I doing wrong? Can someone please point me in the right direction?

Paul