OpenPDFSign is a free open-source tool to digitally sign PDF files for free. It lets you sign multiple PDF files right from the command line. There is a very simple command that you have execute and you can even turn that into a batch file. As an input, it takes private key and certificate file that it uses to sign your PDFs. Additionally, you add customized signed stamp on any of the page in the PDF file with a logo.
This tool OpenPDFSign meant to be installed on servers. It will make sure that every PDF file that is served by your server is signed and the digital signature of that PDF file can be verified as well. Mostly the tools that can digitally sign PDFs are paid. So, in case you are looking for a free solution with full controls, then OpenPDFSign is you need. However, you need to have some basic knowledge of command line to use it.
Digitally signing PDF files is the same as ink signature on a paper but it’s more secure. In this, a unique key and certificate are placed inside a PDF file which can be detected and verified via different PDF readers out there. Digital signatures ensure that a PDF file has not been modified by a third person. Another advantage of digital signatures is that it helps you make sure that the PDF has come from the same person who created it.
Digitally Sign Unlimited PDFs using your Own Certificate for Free:
OpenPDFSign tool is written in Java. And thus, you will need latest Java Runtime or Java JDK to run it. The binary release of this tool which is a JAR file can be downloaded from here. If you have Java install, you will see the Java icon on it as you can see below.
Now, you open the command prompt in the folder where you have downloaded the OpenPDFSign file. You can also place the PDF file in the same folder where the JAR file is.
Now, along with OpenPDFSign and the into PDF files, you need two more files; the private key file and then certificate. You can generate those files using OpenSSL. To generate those files, you just use this command.
openssl req -x509 -newkey rsa:4096 -keyout ./key.pem -out ./cert.pem -sha256 -days 365
Copy-paste the key and the certificate file in the same folder where the input PDF file and OpenPDFSign JAR file is. So, now, you have total 4 files as shown below.
To sign the PDF file, you just execute the command in the below mentioned syntax. It will take a few seconds and will sign the PDF file for you.
Syntax: java -jar ./open-pdf-sign.jar -k Private_Key -c Certificate -i inputPDFFile.pdf -o output.pdf -p Passphrase --page -1
Example: java -jar ./open-pdf-sign.jar -k ./key.pem -c ./cert.pem -i input.pdf -o output.pdf -p 1234 --page -1
Open the generated signed PDF file and you will find the attached signature on its last page. It looks like this.
Now, you can keep using this tool in this way to sign unlimited PDF files digitally. You can also turn this command into a batch file and use it to bulk sign PDF files for free. Aside from this, if you want to customize the digital signature mark in the file then you can do that. You can manually supply the logo, organization name explicitly in the command above. The modified command will look like this.
java -jar open-pdf-sign.jar -i input.pdf -o output.pdf \
-c certificate.pem \
-k privkey.pem \
--page -1 --image mylogo.png \
--hint "You can check the validity at signaturpruefung.gv.at"
In this way, you can make use of OpenPDFSign tool to bulk digitally sign PDF files at no cost. For this, post, I have used it on Windows but the process and the commands are same for Linux and macOS. This tool is fully open source as well and you can take a peek at that on its GitHub page.
Wrap up:
There are many signing tools but a totally free and open source for PDFs only is kind of hard to find. But all thanks to this OpenPDFSign projects that I found on GitHub; you can now sign unlimited files for free. it supports all kinds of PSD files and is actually pretty fast as well. So, try it on your end and let me know what you think about it.
