Need to store sensitive documents in the cloud. Expert opinion desired

How sensitive? What threat models are you trying to protect against? 

I would replace those Windows 7 machines. They're not getting updates and are a major security risk.

a) Would you be satisfied with the security of those same documents if they were laying on the desk of some random employee, paid mininum wage, at a company you know nothing about?  (That's essentially what will happen when they are "put in the cloud.")

b) What will you do if the internet becomes inaccessible and those documents must be retrieved immediately?

c) What will you do if there is a rack fire at the "cloud" provider, destroying those documents without hope of retrieval?

d) This one you need to ask your legal department.  Is putting those documents in "the cloud" going to be legally defensible if there ever an issue involving them?  Will that be considered "best effort"?  Will it comply with your state's privacy laws, if individually identifiable information is contained within?

e) Facebook considered losing the personal information of 533 million users no big deal and no penalties will be paid.  Will your "cloud" provider pay off if those documents are stolen?  At your defined valuation?  Without quibble or "mandatory arbitration"?

You can use 7-zip to zip the files with an encryption key and password only your buddies know, then save the zipped file in the cloud.

Actually you could use any of many file encryption tools.  I mention 7Zip, because its free and it works reliably.  see:  https://www.7-zip.org/

Hello Fred Hakim:  Thank you for your very interesting answer.  Most of the files are pdf files.  So, if I understand your answer, the members could create pdf files, encrypt them and then move them using something like OneDrive.
Question:  I want to make sure I understand :   To edit a file the members would have to download the file  from the cloud, unencrypt it , modify the file, then encrypt it again and move it back  to the cloud?  Please provide as much detail as possible.  I am not a computer expert.

Thank you all for your help.

You are welcome.  I hope you have enough information to test out the various alternatives and select the most effective solution for you and your team.