Viewing documents without downloading

Hi I’m setting up my new My Cloud EX2 Ultra and am having some trouble viewing documents. Photos can be viewed online but if I click on a document name (both excel sheet & pdf) they are automatically downloaded not opened.
Using both Mac & PC. I expected the NAS drive to work like dropbox, once you open a document, any changes are automatically saved/updated. Is there anyway I can operate the NAS drive like this?

Media files can be accessed and viewed because the NAS has ability to process media files. Excel and PDF files need another program to view them and these programs are not built in – they need the programs on your PC and why they are downloaded. This is all explained in complete user manual; refer to it.