Storing your iWork – Pages, Keynote, or Numbers – documents on iCloud is convenient as it allows you to work on the same documents on your Mac, iPhone, iPad or via iCloud.com in your browser. Documents are readily available where ever you are or with whatever device you have.
If you have older iWork files that are stored locally on your Mac, you can move them easily using the File Open dialog window.
Launch Pages on your Mac. You must be running the latest version or a version new enough that support iCloud. Select from the menu File > Open for the File Open window. Click on the “iCloud” button at left-top corner of the window to display the iCloud folder.
To move your existing Pages documents in your Mac to iCloud, simply drag-and-drop the files from Finder to the File Open window. Repeat the same for Keynote and Numbers.
iCloud folder is application specific, thus you can only drag-and-drop Pages documents into the File Open window of Pages app. Likewise for Numbers and Keynote.