I want to convert in C# a PDF file to .jpg's files. It should be one .pdf page in one .jpg file. Can anyone explain me how to do it?
You could use Ghostscript to convert PDF to images.
To use Ghostscript from .NET you can take a look at the Ghostscript.NET library (managed wrapper around the Ghostscript library).
Code sample of how to rasterize PDF to image can be found here: GhostscriptRasterizer sample
If you don't want to mess with Ghostscript API and need a quick working solution you could use Super Pdf2Image Converter to generate the images from a PDF in high resolution. It's available for both 32 and 64 bit and is very cheap and effective. I'd recommend you to try it.