Question:
How do I get the DPI at JPEG image?
Answer:
You would first use this forum post to confirm that the image is in fact JPEG image, by looking for DCTDecode.
https://groups.google.com/d/msg/pdfnet-sdk/4i-IQuGfwb0/rD9GnI-r0gAJ
Once you have confirmed, you want to get the raw stream, using Obj.GetRawStream(true)
https://www.pdftron.com/api/PDFTronSDK/dotnet/pdftron.SDF.Obj.html#pdftron_SDF_Obj_GetRawStream_System_Boolean_
That gives you the JPG image in memory, which may or may not contain the DPI metadata, but either way you would load/open in any image processing API you have available to you.