create a new page, 

upload your document to the server into a special folder, 

add that folder to the htaccess via these directions https://developers.oxwall.com/forum/topic/47116

then add the link to that folder/file to the custom page

then add the link to the new page to your groups post

OR 

upload the file to the server as before

and add the link directly to the post in the group

OR

you can try this.. 

https://developers.oxwall.com/store/item/1277

OR this (i dont know if this works anymore - puru has left oxwall  and is not updating his plugins anymore)

https://developers.oxwall.com/store/item/361

Thanks very much Dave.

Actually, we wanted each member to able to share his/her file. But users do not have access to the server/htaccess file. So, is there a way to do this? Thanks once again.

create a folder for each user, 

then password protect that folder via cpanel,

then you will need to add that users folder to the htaccess for them,

then you give the user their password login info,

then you write a quick php script to upload the docs,

then the users can use that url to link the file. 

OR

you can use a third party service, 

let them upload the file to that service,

then they can link to it via the other services link