Because Android WebViewdoes not support PDF functionality . And WebViewcan not display PDF files.
The URL you posted in your comments displayed the pdf file in your own web browser, which is why it uses Google Docs .
pdf -, Google Docs URL- pdf, .
Update:
pdf , -.
WebView webview = (WebView) findViewById(R.id.webview);
webview.getSettings().setJavaScriptEnabled(true);
String pdf = "http://www.xxxx.com/xxxx/xxxx.pdf";
webview.loadUrl("http://docs.google.com/gview?embedded=true&url=" + pdf);