Where did changes I made to the document/workbook go?

Imagine this scenario:

You create a new VSTO solution (Word document or Excel workbook) and you run the solution. Respective application will open and you make some changes to the document/workbook - maybe you add a few pictures, change text formatting, etc. Once you're done you save the document and close the application. You might be surprised to see that the changes you made are not reflected in the document designer inside Visual Studio. Don't panic - changes you made are not lost.  Reason you won't see any changes is that there are multiple copies of the document/workbook.

When you run the solution Visual Studio copies the document from solution directory to \bin\debug\ or \bin\release\ folder. This is the document that will open when you run your VSTO solution and this is the document where you made all your changes. But when you close the application and return to Visual Studio, the document opened inside VS is from your solution directory.

So, instead making changes to the document when you F5 the solution, do your changes inside Visual Studio  (if you're customizing an empty, new document) or create your solution based on existing document which was customized outside VS. Or you can open the document in \bin\debug folder and copy/paste the changes you made.