httpwebrequest request = (httpwebrequest)webrequest.create ("http://www.ic-on-line.cn/iol/datasheet/tc74hc697_767537.pdf");
request.maximumautomaticredirections = 4;
request.maximumresponseheaderslength = 4;
request.credentials = credentialcache.defaultcredentials;
httpwebresponse response = (httpwebresponse)request.getresponse ();
console.writeline ("content length is {0}", response.contentlength);
console.writeline ("content type is {0}", response.contenttype);
stream receivestream = response.getresponsestream ();
.....
这里把 receivestream 写为文件就行了.