The Federation of American Scientists has published on its website the National Security Agency's Redacting with Confidence: How to Safely Publish Sanitized Reports Converted From Word to PDF. I suspect that the practices set forth in the report can be applied to WordPerfect documents as well. Now there's no excuse for sending out pdf documents with metadata.
It is questionable whether it is unethical to read the other parties' metadata when a document has been sent to you by the opposing side. However, the sender should consider the malpractice implications of failing to sanitize documents.
Hat Tip to Boing Boing.