For some reasons, users would like to sign a PDF document with digital signatures. Using digital signature, no matter visible or invisible, adds metadata on PDF document and keeps its integrity. Then, what is the difference between visible signature and invisible signature on PDF. The visible signature shows that PDF document has been signed and the signer information is visible on PDF page. Thus, always used for copying and archiving. Conversely, the invisible signature omits the visible signer and can only be viewed in the signature tab within the navigation panel that is always on the left side of PDF viewing applications, like Adobe Reader, Adobe Acrobat, etc.

Is it possible to develop a C#.NET application that allows to sign PDF document with a digital signature that is invisible? Of course, yes. Besides adding visible signatures on PDF document, like text signature, image signature, and the two types both, XsPDF .NET PDF Digital Signature SDK for C# also supports creating and inserting invisible signature to PDF file page. You can choose to add visible or invisible signature to PDF file based on your requirement. For the invisible one, created signature will not appear on physically printed PDF documents.

Add Invisible Signature to PDF in C#

You can add visible or invisible digital signature to the PDF document in C# programming projects without any problem. Please directly specify the invisible signature to be added to PDF file page. See the following C# example for more details.

//Load the pfx certificate file with password
PdfCertificate cert = new PdfCertificate("demo.pfx", "your-password");
PdfSigner signer = new PdfSigner("demo.pdf", cert);           
signer.ContactInfo = "your contact information";
signer.Location = "your location";
signer.Reason = "your reason";
signer.FieldName = "XsPDF Sign";
signer.PageId = 0;
//Set width and height of signature area to 0
signer.Rect = new Rectangle(0, 0, 0, 0);
signer.SignatureAlgorithm = SignatureAlgorithm.SHA256;
signer.SignatureShowingType = SignatureShowingType.Description;

signer.Sign("invisible-sign.pdf");

Notice - If you use the free trial version of .NET PDF Digital Signature SDK, the sign field is showing "XsPDF Signer Trial Version".

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