When we upload any office document in sharepoint document library do sharepoint make any changes or add properties to the document?
It depends upon the SharePoint application and also the Office version (e.g., use of Document Parsing & Record Center Sites - where doc properties are related to the site content) - see this post for an example & more details: The Case of the Missing Office Document Properties in SharePoint 2010.
As far as I know SharePoint is not making any changes when you upload the document it just extracts the properties matching columns in the library, but when you access the document from a document library SharePoint will add/update these properties in the document.
But the result is the same.
Your document will not be unchanged when you retrieve it later.