Hi kath57,
Thanks for using Microsoft products and posting in the community, I'm glad to offer help.
According to your description, since Dropbox and Microsoft Office are products of two different companies, their production technologies are different, so there may be a little conflict when used together.
In your case, to better understand and reproduce your situation, could you please indicate how you "open a document in word or powerpoint" that saved in your Dropbox? Maybe you can try to download the files from Dropbox to local disk and then open
them with Microsoft Office.
Besides, if you have encountered any error message when trying to access files within Microsoft Office, could you please attach a screenshot of the error message? Your cooperation and understanding will be highly appreciated.
By the way, if you want, I'd like to recommend you to use OneDrive, which is the best practice to use Microsoft Office and cloud storage to work together.
Looking forward to your feedback.
Best Regards,
Arck