If you already have the idCAT certificate installed on your computer, you must open the pdf with Adobe Acrobat Reader and follow the following steps:
Certificate installed in Chrome or Internet Explorer
1 - Go to Tools
2 – Select the Sign digitally option
3 – A blue rectangle will appear that you will have to move until you bring it to the space where you want the signature to appear.
4 – It will automatically ask for the certificate you want to sign with.
5 – And finally the signature will look like this:
Certificate installed in Firefox
You must follow the same steps as in the previous point, but first you will have to export the idCAT from Firefox, you will find the steps in the following FAQ: