noun, noun which may take the genitive case particle 'no'
confidential; personal; to be opened only by addressee
Used on envelopes or documents to indicate that the contents are private and should be opened only by the named recipient.
封筒に「親展」と書いてある。
The envelope is marked 'Confidential'.
親展の手紙が届いた。
I received a confidential letter.
confidential letter
registered mail to addressee in person; PD PR
From 親 (intimate, personal) and 展 (open, unfold), literally 'to open personally'. The exact historical derivation is uncertain, but the compound has been used in Japanese correspondence for many decades.