Google Docs has no native option to display hidden formatting marks.
Stating the obvious, this means that Google Docs can only display printable characters...
This also means that in order to make invisible marks visible in your document, it has no choice but add visible characters to your document. i.e. add ¶, as a real character, at the end of each paragraph. Be aware that...
This ¶ doesn't replace your paragraph mark, but is added before this invisible paragraph mark
Deleting the ¶ symbol will not delete the invisible paragraph mark itself (you'll have to press delete twice).
Your original document will be modified by Show, as real characters will be inserted inside your text. However...
Show can remove all the marks it added to your document
Undo function, along with Google Docs Version Control features, guarantee that no matter what, you can always revert your document back to its original state.
In these pages you'll find more information about some Show features, as well as a few reminders on Google Docs and text edition in general...