Instead of using what you're calling a 'Master word document', you should be using a Word template. Templates are used for creating documents. As for the variable text, you could add a custom document property, like 'AccountName' to the template and, when you create a new document from it, simply add the relevant details to that property. Then, in the document itself, you could use field coding to conditionally show/hide the related content. Such fields might be coded as:
{IF{DOCPROPERTY AccountName}= "Company A" "Conditional content for Company A"}

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues.

Alternatively, if whatever you're producing relates to an Excel workbook or a DATABASE that can supply the company data, you might use a mailmerge.

Without knowing more about your specific requirements, it's difficult to give tailored advice.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]