The Preflight Verifier
From Scribus Documentation Project
Check for overflow in text frames -- It can be easy to fail to see the overflow indicator in the lower right hand corner of a frame somewhere.
Check for transparencies used -- Especially important if you are exporting to PDF 1.3, since this does not support transparency.
Check for missing images -- Any empty Image Frame will trigger this. It may be that no image was ever loaded, or it could be that when a Scribus file was loaded an image could not be found where it should have been.
Check image resolution -- For production of a PDF for the web, 72 dpi is adequate, for most work that will be printed, 300 dpi is probably a desirable resolution. For some high-quality professional work, much higher may be desired. The scaling of images in frames will determine this end resolution.
Check for placed PDF files -- what you are checking for is a PDF loaded into an Image Frame, where it will be rasterized or converted to a bitmap. Its resolution may be less than ideal. See PDF Export to learn how to minimize this problem. Also see elsewhere in this manual about other workarounds for including PDFs in your Document.
Check for PDF Annotations and Fields -- These features are mainly intended for using them on a computer. They may cause issues in professional printing.
Running Preflight Verifier
If you have checked in your Preferences to automatically check on print and export, you will surely run it then. You may also choose to run it at any time by clicking the "traffic light" icon on the Toolbar.
The Preflight Verifier uses 3 different colors to indicate the status of an item in a document. Green means no problems. Red is used for errors like missing images, and yellow is warning that will be triggered for problems like overflowing text frames. Note that an error message from the Preflight Verifier need not necessarily mean there is problem. However, to be sure you should at least check it by clicking on an object that's showing a yellow or red dot. Scribus will select this object automatically, so that inspection is easy.