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The PdfDocumentSigner class allows you to sign and save a document. You can apply a signature to an existing form field or create a new signature field and sign it. You can also sign documents that already have a signature. PDFpen allows you to export, edit, and sign PDFs without effort. You can choose to add a hand-written signature with the trackpad or a digital signature. What’s more, you can edit text, correct content, fill out forms, and extract important information by OCR to turn a scanned document into a selectable and editable file. The cryptographically informed reader might ask how it is at all possible to modify a file without invalidating the signature. After all, hash functions are supposed to prevent exactly this kind of thing. The answer here lies in the incremental update feature of the PDF standard. The specification allows for updating files by appending data to the end of the file, keeping the original bytes in place. These incremental update sections can create and modify existing objects in the file, while still preserving the original version in some form. Such changes are typically opaque to the user that views the file. The byte range attached to the signature ensures that the document hash can still be computed over the original data, and thus the integrity of the signature can still be validated. Last but not least, EasePDF is another popular free PDF signing tool. In addition to electronic signatures, it offers multiple PDF conversion options that allow you to change your PDFs into Word, Excel, and other formats. You can quickly get started by clicking on the function you are interested in from their homepage. Signaturely has a variety of other useful features.
