.udf is the document format used by the UYAP Document Editor. Petitions and filings submitted through UYAP, the Turkish courts' e-justice system, are mostly prepared in this format.
There are two ways to open a .udf file. The first is the UYAP Document Editor, which has to be installed on your desktop. The second is udf.web.tr: it opens the file in your browser and needs no Java.
udf.web.tr processes the file in your browser and does not upload it to a server. After editing, you can save it as .udf or convert it to PDF.
Opening a file does not tell you who sent it. A .udf file does not identify its sender by itself. For that, the document needs to be verifiable.