When tapping a document, it opends in Preview mode by default. In this mode, it is convenient to quickly view the contents of the document, as well as scroll through it.
To start editing the document and unlock the editing tools, tap the button 
at the bottom right.
Once you have are done editing the document, tap the 
button on the toolbar. The document will be saved and opened in Preview mode.
To exit the editor, tap the 
button.