Q: how can i add documents to the icloud from my computer

The following works with Word or Excel files as an example. Whether you will be able to access other files on your iPhone or iPad will depend on the type of file and what applications are installed on your iPhone/iPad.

One method: log in to https://www.icloud.com/ and open iCloud Drive there and then click the Upload icon near the top (looks like a cloud with an up-arrow).

Another method: install iCloud For Windows: https://support.apple.com/en-us/HT201391