Keep in mind that QuickBooks Online (QBO) has a 25MB size limitation when uploading documents. Also, make sure that the name of the file doesn't contain any special characters. Currently, these are file types that you can upload into the program:
PDF
JPEG
PNG
DOC
XLSX
CSV
TIFF
GIF
XML
If you're already using the supported file type and size, you can try uploading the documents using a private or incognito window. It could be that the stored cache and cookies in the browser are the root cause of this strange behavior. Here are the shortcut keys:
Ctrl + Shift + N for Google Chrome
Ctrl + Shift + P for Firefox & Microsoft Edge
Command + Shift + N for Safari
If it's now working well, go back to their regular browser and then, clear its cache to delete the accumulated junk files and start over fresh. You may also switch to other supported browsers to further isolate the problem.
Lastly, here's an article that you can read to learn how to attach documents to your transactions and customer/vendor profiles: Attachments in QuickBooks Online.
Drop me a comment below if you have any other concerns or additional questions. Have a good one.