Just to add a little to what utnik said, printing to a PDF is not the same as exporting to a PDF.
When you print to a PDF - via Scribus' Print function - that's no different than printing to a page except that the output goes to a PDF rather than a physical page. That might sound odd - or obvious - but it's not the same as exporting to a PDF. The print function is not the same as the export function. They work differently.
Printing to a PDF creates the PDF by using a printer driver to take the data from the print function and pushing it to a file rather than a physical printer. What you end up with is, basically, just a graphical image of the document rather than a real PDF. Printing to a PDF will not preserve layers, or bookmarks, or fields, or scripts, or anything like that. Think of the print function as - at best - just creating a photocopy of the document rather than the all-singing all-dancing document itself. On the other hand, exporting to a PDF creates a PDF the "proper" way with all of the features intact.
Many people have reported that the print function doesn't work properly - and quite a few have recommended that it is removed. The export function should always be used to produce output from Scribus whether the resultant PDF is to be physically printed or not. At the very least it can be used as a final proof before printing.