I had to import an older project (in .Net 2) into Visual Studio 2013, it makes use of the Microsoft Primary Interop Assemblies.
Visual Studio said that I need to add references to the project. Now I went and did some reading and apparently Microsoft has only released the PIA for office 2010? (I have Office 2013)
Now what I would like to know is.
.Net 2 (at the latest) and Windows 8 comes with 4.5 and not 3 (by default) and most new computers are going to have Office 2012 or 2013.PIAs are a historical artifact, required only by old .NET versions (before v4). They have been thoroughly and elegantly replaced by the "Embed Interop Types" feature, also known as the "No PIA" feature. Supported since Visual Studio 2010, you'll find it back in the Properties window when you select a reference assembly. It defaults to True. A good video that covers the underlying technology is available here.
Which is the reason that Microsoft doesn't publish the PIAs for Office 2013, they expect you to embed the interop types instead.
The feature is very desirable, it avoids your customer having to install the PIAs on his machine and for you to include them with your installer. Solving the issue when neither takes care of it, an entirely too common mishap. In addition, the PIAs for Office are very large, the great advantage of embedding the interop types is that your assembly only contain the types that you actually use. Many megabytes reduced to a few kilobytes.
The workflow is a little different. Instead of adding a reference to the Microsoft.Office.Interop assemblies as available in the Add Reference dialog, .NET Framework tab, you now use the COM tab. And pick, say, "Microsoft Excel 15.0 Object Library" to generate the interop types for a program that uses Excel. If you load an old project that previously used PIAs then just remove those reference assemblies and add them back from the COM tab.
Do note that a feature is lost, intentionally targeting an old version of Office that you don't actually have installed on your dev machine is more difficult. If that's a requirement then you still need the PIAs for that version, force the Embed Interop Types to True in the Properties window. Actually doing this is questionable, Microsoft has a hard time keeping new Office versions completely compatible with old versions. They've kept it up for 15 years now but it has been running out of steam. A worst-case scenario is targeting a newer version than you have installed on your machine, that's liable to make your program crash with very hard to diagnose exceptions like AccessViolationException.
Do note that you have to make small modifications to your code to allow it to work. The synthetic "XxxxClass" classes are not embedded, just the "Xxxx" interfaces. Simply remove the word "Class" from the new statement.