I have finished my project and need to convert it into a PDF file. How can I do it?
I'm going to answer this but you should really do a quick search first before asking something like this, or at least have a look though the manuals at: http://www.qgis.org/en/documentation/manuals.html
If you want a PDF you need to first create a composer window,
File->New Print Composer... Once you have a new print composer you can add a map (
Layer->Add Map..) and add it by dragging an area in the composer.
You can then export the composer to PDF using
File->Export to PDF..
Unfortunately, Qgis, in versions prior to 3.0, does not export the PDF into Layers, only in bitmap. In version 3.0, the user can export in PDF as it does in AcGis. The QGis has evolved satisfactorily in this latest version. He has solved many bugs and is leaving the composer with an interesting quality.