A digital signature is like a paper-based signature – each giving you a method for adding a unique identifier to a document. Both provide you with an easy way to show a person signed a particular document at a particular time, but a digital signature goes further as it can ensure that the document content has not been tampered with after it was signed. In addition, the document version history can let recipients see when a document was signed and see when any changes were made to it. This revision history is encrypted and stored inside the PDF and can be viewed at anytime via the Signatures pane.

To digitally sign a PDF document, you must have a digital ID, and to verify other people’s signatures, you must first share your digital IDs and add each other to your Trusted Contacts list.

Most Digital Signature software are not free, but I will introduce you some free software developed in JAVA and DOTNET with the power of iText and iTextSharp.

• PDF Digital Signer, it is portable version developed by RubyPDF
• jPDF Tweak, java version, Open Source.
• JSignPdf, Java version, Open Source.

Want to develop your own? please visit How to sign a PDF using iText and iTextSharp.

news2paper is WordPress plugin aims to produce a newspaper-like version of your blog, free of charge.
The news2paper web service is used to generate a printable PDF file that is then uploaded to your blog. All communication between your blog and the news2paper web service is encrypted to ensure that the news2paper contains full posts while your Atom or RSS feed may still contain only teasers.

You can choose your theme from our set of standard choices or you can get your own theme at http://www.news2paper.com/custom.

And it is very easy to install,
1. Get an API key at http://www.news2paper.com/wizard/single
2. Upload the unzipped contents of the plugin archive to the /wp-content/plugins/ directory.
3. The plugin files should now be in a subdirectory called news2paper. If not, create this directory and move the files there.
4. Activate the plugin through the ‘Plugins’ menu in WordPress
5. Configure the plugin through the ‘Settings’ menu in WordPress, especially providing the API key.

Do you want a free/open source application that can combine, split, rotate, reorder, watermark, encrypt, sign, and otherwise tweak PDF files?
If sure, please have a look jPDF Tweak.

With jPDF Tweak, you can use it to make printable booklets from your PDFs, to add PDF bookmarks, effects (page transitions), to combine multiple PDF files, to watermark them, to rotate pages that do not fit, to attach files to your PDF, to encrypt and sign your PDFs, to change metadata (like author or keywords), and much more.
jPDF Tweak uses the excellent iText library for manipulating PDF files, and JGoodies Forms for the GUI.