The Dark Pictures Anthology: Man of Medan has 63 Collectible Locations - 50 Secrets & 13 Pictures (6 Black Framed Pictures, 6 White Framed Pictures, 1 Gold Framed Picture).

Finding all Collectibles unlocks the following trophies / achievements:

- For a dollar he told me his secrets - Found 25 Secrets!
- Quite a lot of deaths that night - Found all of the black framed Pictures
- Possible Futures - Found all of the white framed Pictures
- Secrets! Lies! Conspiracies, man! - Found all Secrets!
- Ghost ships are just a myth, right? - Found all Pictures

The following collectibles can only be found in Shared Story (Online Co-Op) or in Curator's Cut Mode (Pre-Order Bonus):

In Chapter "Glamor" Girl: Secrets #15, #31, #17
In Chapter "Matters of the Heart": Secrets #46, #12, #33

There is no matchmaking so you need a friend to help you out. If you have the Curator's Cut Pre-Order bonus you can do these solo though.

The pause menu shows all collectibles you have found. You can get missing collectibles via chapter select which unlocks after the first playthrough.
It's best to follow this guide first in Theatrical Mode, then get the collectibles exclusive to Shared Story / Curator Mode. Get the rest via Chapter Select. Many chapters lock out each other so you can't get everything in one playthrough.