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Emilee Crocker 3Emilee Crocker 3 

How to change permissions on custom object's notes & attachments?

All my users have access to post notes and attachments on a custom object except one and I cannot figure out what permission set is off.
Rahul KumarRahul Kumar (Salesforce Developers) 
Hi Emilee,

Notes and Attachments are a little funky in that there aren't permissions associated with them directly. Instead, the permissions are determined based on the parent object which can cause some issues.

I suggest instead of Notes & Attachment you use Chatter Files and upload your attachments to chatter files. 
Loading to Chatter files will let you take control on who can and cannot modify the attached file, it will also provide information on updates to the files if any on a real-time basis to all the users following that record.
Please refer the below link for reference. Let us know if it helps.

Please mark it as best answer if the information is that question is removed from an unanswered question and appear as a proper solution.

Rahul Kumar
Pankaj PPankaj P

Hi Emilee,

Check information below :
To delete a note or attachment, you must be the owner of the note or attachment or an administrator with the “Modify all Data” permission. Note ownership is determined by the owner field. Attachment ownership is determined by the created by field.
Record owners (except Portal users) can delete attachments on records.
Notes and attachments marked as private via the Private checkbox are accessible only to the person who attached them and administrators. For administrators to view private notes and attachments, they need the “View All Data” permission; to edit or delete them, they need the “Modify All Data” permission.

You also need to have “Read/Write” access permission to parent record to be able to edit or delete Notes or Attachments.

Reference :

Hope this helps you.