For some reasons I need to convert pdf documents to images frequently, but the image sizes of them aren't equal, and I have to resize these images subsequently. Is there a way that I can convert these pdf files to a image size I specified? I'm a Windows user, so I need Windows applications, thanks in advance.
I am not sure what you need to do. For Windows I have this software Dysprosium (Edition 2015). One feature it has got is to convert PDF's to image files (png, gif, tiff, bmp, but jpg not sure) depending to the users' choice. You can decide on scale before-hand. You are able to convert images back to PDF files, too. It is free and works on Solaris, MAC, Linux. You need free Java already in your computer to run it. It does not need installation. Put it in the bin if you do not need it anymore. Use PDF to Image button. Software is versatile and has other features that you might use; for example, change default size and zoom of all your PDF files.
There is a help page here to utilize the software.