Open a lead from Lead view
Tap on Notes to to see the notes if there
Tap on Add Note to create a new note. It has option to add file to attach file from mobile or take a photo from mobile to attach
Tap on Save to save the note
How to attach a document or file for a lead?
Next tab is Files to view file or add attachment
Once Tap on Add attachment button, it has the option to select the file type, it might be File, Link or note