Lawyers use the same kinds of encryption found in any other corporate environment: TLS for secure web browsing, private messaging and virtual private networking, and FileVault or BitLocker for full-disk encryption.
While lawyers often deal with confidential documents, most lawyers (and clients) lack the technical skills to manually encrypt documents using tools like PGP. Moreover, securing individual documents with PGP provides little benefit, since documents are not stored in a location that is accessible to untrusted users anyway.
The exception is email. It would be nice if lawyers encrypted their emails, but even though security experts have been calling for this for years, it rarely happens. Some lawyers might use the encryption feature in Microsoft Outlook, or attach confidential documents as password-protected PDFs or archives. More commonly, if a document is particularly sensitive it might be distributed only in paper format, with a legal obligation on the recipients not to make copies.