Could you please confirm which mobile phone you are using?
Is it Android or IOS?
In order to save the pdf file on Document Cloud you first need to download the Adobe Acrobat Reader mobile application in your phone.
> Open the pdf file
> IOS- Right hand side bottom corner click on the icon highlighted below-
>Android- Right hand side top corner you will see icon for settings
>click on that and then Save to Document Cloud.
Let us know if that helps.