I’m frequently noticing questions regarding problems with paper size and pdfLaTeX, for instance Bug 215561 and Bug 244043 on Launchpad, yesterday on mrunix.de and also yesterday on matheplanet.com.
If you manually change \paperwidth or \paperheight or use the landscape option of LaTeX classes like for instance article it could happen that the paper size of the resulting pdf differs.
A solution is to adjust the \pdfpagewidth and \pdfpageheight values:

\setlength{\pdfpagewidth}{\paperwidth}
\setlength{\pdfpageheight}{\paperheight}

To use the geometry package could correct it too. But if you use it together with gmeometric, it may still be necessary to adjust the pdf page size like written above, see for instance goLaTeX.de.