What do I need to open a document ending in " .pub"

You need Micosoft Publisher.

I have seen a suggestion that you can send a .pub file here

https://www.pdfonline.com/convert_PDF.asp.

to get it converted for free to pdf if it is less then 2MB. Bigger files need to be payd

The link works but I have not tested the conversion.

I have just discovered http://www.zamzar.com

Everyone on the net seems fixated with converting the .pub to .pdf. But this option allows you to convert .pub to .doc. and .docx. Which is far more handy if you want to edit or work with the document!