Q: How do I open a pdf for reading through a URL?
You could use standard .NET Framework API (or some C/C++ network
library in case you are using PDFNet for C++) to download a remote file
in memory or to a temporary file.
Something along the following line:
FileStream sourceFile = new FileStream(@"...", FileMode.Open);
long sz = sourceFile.Length;
byte getContent = new byte[(int)FileSize];
sourceFile.Read(getContent, 0, (int)sourceFile.Length);
You can then open PDF memory buffer as illustrated in PDFDocMemory