There was an article written by Sean Byrne on myce on 17th of April 2014. He describes and asks whether Microsoft when synchronizing local data with OneDrive for Business changed these documents. He first achieved once by means of MD5Summer proof. Then he tries to deal with differences. Then he finds that the consumer product OneDrive no changes take place. A more detailed, in-depth analysis of the documents is only sketched. And then he leaves the open question, why does this Microsoft?

The answer is actually quite simple.

With consumer product OneDrive the data is stored directly in a cloud database, with  OneDrive for Business is then still either the SharePoint Server 2013 (on premise) or SharePoint Online intervening before the data in SQL Server are stored.

Such messages make but very quickly the round, even "negative" news travels very fast. And no one goes the matter to the bottom.

For the whole already exists with the predecessors of products such as SkyDrive Pro and SharePoint Workspace 2010. And it actually has nothing to do with the client products. As mentioned above, the client products are only responsible for the transport, presentation, and more. Only when the data arrives at the destination, this data from SharePoint will be worked up.

Right, SharePoint Server 2013 or SharePoint Online. Or SharePoint Server 2010. It is called namely Promote and demote document properties and is described here. And the article was published in May 2010!

If an Office document, which is present in the original and after synchronization, manually transformed into a zip file, then the contents can be compared.

It becomes evident that the document restructured, the original content is not changed!

This means:

Yes, files are modified by a process. And this process can not switch off with SharePoint Online in the cloud. An administrator or a developer can change this behavior in the OnPremise variant.

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Summary:

Is my content being modified or changed?
No. The context of content is not modified.  Limited metadata is added to content to support advanced document management scenarios and preserve user experiences such as synchronizing document properties across its parent folder to enrich discovery or updating links when a link has changed.  The functionality behind this has been in the product for several releases and is designed to synchronize important metadata between a document and a Document Library including OneDrive for Business. (fka MySite)

Are there any document types where these properties do not apply?
No.  These properties apply to all documents in a SharePoint Document Library including OneDrive for Business.

Do these properties contain any personally identifiable information?
No.  There is no exposure of any personally identifiable information as part of this functionality because the only properties that are synchronized are ones that users have already created. 

Do these properties make documents individually traceable by Microsoft?
No.  We copy properties from the user’s document library into the document itself to be used by client side editors.  This may result in content being added to the document.  If the file is HTML or a similar type, the content being added will also be HTML. This may look like code, but it is just properties copied from the user’s library into the document itself.

Are Document Promotion and Demotion Document Properties new?
No.  The concept of Document Property Promotion and Demotion has existed in several releases.

Can I disable or turn off Document Promotion and Demotion?
Yes and No. In Office 365, Document Promotion and Demotion cannot be disabled.  For on-premises customers, developers can implement the SPWeb.ParserEnabled property to effectively disable Document Promotion and Demotion.