It's really interesting to see that there are foxit employees on the list of committers. I assume that means that it was initially a fork of the foxit PDF reader?
"Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com"
https://pdfium.googlesource.com/pdfium/+/master/core/src/fxg...
"/\ * * Copyright (c) 1998-2000, Microsoft Corp. All Rights Reserved. * * Module Name: * * Gdiplus.h * * Abstract: * * GDI+ Native C++ public header file * \/ "
I saw no evidence in the project showing that this file has BSD-style license.And since it is part of the windows SDK, it is nearly impossible to be BSD-style licensed.
Maybe it was included from foxit's code or other codebase but it is better to be put into the thirdparties directory due to the license issue.
Edited:
Thanks for pointing out my misconception about Windows SDK.
Agreed it is always nice to have these things in a thirdparty directory, though, but the larger Chromium project actually does appear to have all of pdfium in third_party, which helps keep that clear.