Hi pisosse, hi folks.
If you use the Help menu from SketchUp, then choose the Knowledge Center and then type printing to scalein the search field and then click on the first hyper link, you shall eventually find this:
http://help.sketchup.com/en/article/36227
The 1:50 scale can be set in the dialog window.
When printing, select a pdf printer to produce a pdf file.
Just ideas.