RE: Cannot restore deleted document
Hello Lora,
Please check in Document management parameters form and see if Deferred deletion enabled check box is enabled or not.
You will get this form in below path - Organization administration > Document management > Document management parameters > General.

Set this check box enabled and also set the "Number of days to defer deletion" if required. This field determines for how long system has to keep record of the deleted file. Once that threshold is over, system will automatically physically remove that deleted file.
Once this is set, you can retrieve that deleted file from 3 areas as per the following.
--- Retrieve the deleted file from Deleted attachments tab.

Once you click on this tab, you will see the deleted document is available. Just select the line (in case if there are many such records are already available) and click on Restore.

--- Else; go to System admin > Inquiries > Deleted attachments and click on "Restore" after selecting the line.
--- Else; go to the document from where you deleted the file before. You will see a new tab Deleted attachments is available once you enable the Deferred deletion enabled check box in "Document management parameters" form.

Also you can check below link published by Sukrut. It will give you technical details as well.
https://community.dynamics.com/365/financeandoperations/b/365operationswithsukrut/posts/attachment-recovery-feature-in-dynamics-365-for-finance-supply-chain
Kindly check and update us.
Best regards,
Sourav Dam
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