This is normally a font problem. For example if the document was created on
a different computer with a font that is not available on the computer you
are using when you try and open it.
It does depend on the kind of font used, but often you can turn the squares
in to text by using [Ctrl] + [A] to Select All, then select a font that is
installed on your computer (for example use the Format menu, then Font and
select Ariel or Times New Roman).
I hope this helps.