As explained  in "Is safe?", access to the downloading, uploading or full control (read/write/delete) pages of a bag is defined by the possession of the corresponding link, which acts as a key.
If you get a Whatsapp message containing a download link of a bag containing 20 holiday pictures, you can view them. If you forward the message to 5 family members, then they can view them. Now if they in turn forward the message, the access spreads, exactly like the message itself. No more, no less. Unless someone puts the message and link on the internet, nobody will be able to find it on Google.

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