Many thanks - I have now found the problem
The Word Template had become corrupt and had unreadable content which when populated and created into a word file was pulling through to the file I was trying to convert to PDF
This was not showing as corrupt when opening the word file in SharePoint, but when I open in Desktop App - give an error message - this is what PowerApps was struggling to get past - Recreating and populating a new template to overcome the corrupt file