The digital signature service of confidential documents enables users to directly sign Adobe's globally trusted and state-secret PDF documents on their own computers without applying for a PDF signature certificate from CA or relying on an electronic signature service provider, and there is no limit on the number of documents. Signed documents are automatically time-stamped and support Adobe LTV (signature is valid for a long time). Users can digitally sign all kinds of PDF files to be published at any time to ensure the credibility of the published files and prevent illegal tampering and forgery. The user completes the digital signature of the document on his own computer, and the document does not leave the user's computer, thus ensuring the security of the confidential information of the user's document. The Secret Letter APP also supports encrypting a document with the public key of the encryption certificate of the user who has the right to read the document when signing the document, ensuring that only authorized readers can use Adobe reader to open it seamlessly and normally, and others cannot decrypt the reading document. Of course, Mi Xin also supports the purchase of Adobe Global Trust PDF signature certificate, express delivery of USB Key hardware, and digital signature of PDF documents with Adobe reader.