Your Vault can protect any file and document.
Log in to your Vault, select
in the bottom right and tap Hide photos and documents.
Swipe right or select the Files tab. Select any folder or document you'd like to add to your Private Vault. Submit your selection with the Import Button at the bottom.
Select the document from your Vault and tap on open. LockMyPix shows all installed apps that can open the document.
Mind that LockMyPix doesn't grant access to your encrypted files. If you change a file from outside LockMyPix, just reimport that to have the changed file securely stored inside your Vault.