I'm observing, when I convert a book from PDF to word and navigate it using NVDA, the word document crashes after 30-40 minutes approx.  This doesn't happen if I'm working on document which hasn't been converted from some other format, say when I'm writing a report in word itself
 
  Why is this happening, is this because of NVDA or ms word itself? 
Is this because  of the heavily formatted PDFs that I convert to word and word is not able to handle the formatting? 
All suggestions are welcome --
Regards,
Sociohack