Hi @Yoann.Obry,
there’s no open source project that reads Revit Files directly; the only SDK I know that can read Revit files without using Revit is not open source, and is the BimRv SDK from Open Design Alliance (ODA).
I suppose FME uses that library, but this is only a guess.
I’ve not tested it, so I can’t tell if it’s good enough.
I only evaluated ODA’s DWG library a long time ago, and decided to stick with Autocad core console since it doesn’t expose all the features we needed (namely, the ECW image support is not included, and I got PDFs with white backgrounds instead of ortophotos 
)