Add field linking to related node I have created a Content Type that allow to download PDF files. Meanwhile I want to have this file available in a html version; as in Modules' notes in Drupal's website.

So the easiest way is to: create this html version as a new node, and use node reference field type to link to this node. But this process takes two steps to do it, and within large nodes it will take while to find the referenced node. 

So what is the more sufficient way to add a related node to a content type?
You can do this in two ways.

Option 1:
Add a textarea field to the file content type, with the necessary text options (fe html on etc), and create two various displays via Display Suite. Create a view block with the nid as contextual filter. 
And select the node display for showing.

Option 2:
Install the prepopulate module and setup a rule that after saving the html content type node redirects to file content type with the right url + prepopulate information. Have 2 separate content types. In the file content type add a reference field with prepopulate settings.

Now after you create the node with html information it redirects to the file content form with the reference field filled... So no searching and no hassle.