使用公钥加密文档时，我收到警告：

$gpg --encrypt -r 6B7F10E4 file.pdf

It is NOT certain that the key belongs to the person named
in the user ID.  If you *really* know what you are doing,
you may answer the next question with yes.

Use this key anyway? (y/N)

我将信任级别设置为 4 并签署了密钥：

$ gpg --edit-key 6B7F10E4
gpg> trust
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

1 = I don't know or won't say
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
m = back to the main menu

Your decision?

4 <Enter>
gpg> sign

Are you sure that you want to sign this key with your
key ... (8F3A3A54)

Really sign? (y/N)

y <Enter>
gpg> save

但是在使用该密钥加密文档时，我仍然会收到该警告。我还需要做什么才能将其删除？