If you want to show a friendly name instead of the name of the document on the website, you can enter the friendly name and then the name of the file separated by a pipe character in your import sheet.  For instance, if you wanted a particular document to display "Instruction Sheet" on the website instead of the filename RFD-111.pdf, you would put the following text in the Documents column of your import sheet:
Instruction Sheet|RFD-111.pdf

This same process is used to add alt text to an image.  For example, if you had an image RE112.png and you wanted the alt text to be Tap Collet RE112 you would use the following text in your Images column: Tap Collet RE112|RE112.png.

If you are doing multiple files of one type per item in your import, you will need to separate each file with a semi-colon.  An item with two documents would have its document column created as:

Instruction Sheet|RFD-111.pdf;Instruction Sheet2|RFD-111-2.pdf

Once your file is imported, you can view the friendly names or alt text of any items by looking at the item's "Tags/Attachments" tab in the PIM.

If you are wanting to add a handful of friendly names or alternate text as opposed to a mass import or just want to edit existing names you can do so by looking at the items in the PIM directly and viewing their "Tags/Attachments" tab.

In order to add or edit existing Alt Text or Friendly Names, you simply need to double click in the "Alt Text" or "Friendly Name" columns and change the text to what you'd like.  Once you have finished adding or editing your text, make sure to save your changes to your item. 

You can also right-click the "Alt Text" and "Friendly Name" columns to copy and paste the text between files or use the "Clear Value" option to remove the text completely.