Welcome @Cindy62712 !
First of all, I must warn you - there is no absolutely reliable way to protect a document from forgery. After all, counterfeiters all over the world successfully counterfeit even banknotes - what can you expect from an electronic document? Everything that is displayed on the computer screen can be photographed (PrintScreen or a mobile phone camera) and modified in a graphics editor. Or the document can be printed, the necessary places are covered with a proofreader and a copy is made on a photocopier. Thus, the only guarantee that your contract will not be changed is the honesty and integrity of your counterparty. If you think that your recipient is not decent enough, then it is better not to deal with him.
Now that you know about the potential risks, let’s talk about ways to minimize them.
@robleyd’s recommendation has been quite effective for a very long time. Indeed, Adobe created the PDF format precisely for these purposes - look, read, print, but do not change. But time has passed, and for the sake of additional income, many software manufacturers (including Adobe) have created many paid and shareware tools for editing PDF files. You can use File - Export As - Export as PDF. But this does not guarantee that the contract amount will not change from $413 to $134.
You can sign a PDF file with an electronic signature - if at least one byte changes in the file, the signature will become invalid. But what evidence the court will accept for consideration - an electronic document with a signature or a printed false copy - is not known in advance.
You can see if the Tools - Redact tool is suitable for your purposes
But I reiterate that any document can be forged!
