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Explaining Digital Certificate and Signatures

I'm trying to understand what a digital certificate is and why I need it to sign a document.  I know in the following instructions I can create my own digital certificate for free, but when and how do I use a digital certificate to sign something?  What is a digital signature in comparison to taking a picture of my signature and pasting it.  

Thanks.
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Sam JacobsCitrix Technology Professional / Director of TechDev Services, IPM
CERTIFIED EXPERT
A document signing certificate is used to prove to a user the identity of the creator of a digital resource,
For example, I use my document signing certificate  when I create PowerShell scripts, which adds a digital signature block with a hash to the bottom of my scripts.
This digital signature block does two things:
- It identifies me to the user running the script (if they don't trust me, they should not run the script), and
- It gives the user confirmation that the script has not been modified in any way (or the hash will not match).
so how does it relate with an average person signing a pdf or excel sheet?
Sam JacobsCitrix Technology Professional / Director of TechDev Services, IPM
CERTIFIED EXPERT
If you send me a signed .PDF or .XLS, I know that it was truly sent by you and that it wasn't altered along the way. The same holds for a signed email.
I understand that but what am I using to do that?  Is it a third party program?
Citrix Technology Professional / Director of TechDev Services, IPM
CERTIFIED EXPERT
While you can create your own digital certificate, it's like creating your own SSL certificate.
It will not be trusted unless it's issued by a known CA, like Entrust, Thawte, DigiCert, etc.

The way you sign a document depends on the document.
I use a PowerShell command to sign my PowerShell scripts.
Adobe PDF has a built-in way to sign .PDF files: https://www.youtube.com/watch?v=7ycTI_A9yhE
Microsoft Office files get signed a different way: https://support.office.com/en-us/article/add-or-remove-a-digital-signature-in-office-files-70d26dc9-be10-46f1-8efa-719c8b3f1a2d

Hope that helps.