To create a link to a file within a Drupal site, edit a node and do the following:

Add your text into the text editor.

Select the text you want to use for your link.

Click the Linkit toolbar button.

The Linkit dialog box will open.

Search for Content field = start typing the title of the file you wish to link to - give the system time to run its query.

Once the list of results appears, select the appropriate content. Note how the results are categorized based on content/entity type.

Once your selection is made, the appropriate path will autofill as a relative link in the Link URL field.

Click the Insert Link button.

Your link will now appear. Please note that the link will not be active until you save the node.