Just right now, at export PDF, there is to choose
Automatic
JPG
ZIP
None
This seting has a extreme influence on the size of the generated PDF
All tested set to 200 dpi.
Auto - high -------- 57332 kb
Auto - medium ---- 56851 kb
jpg - high ---------- 10118 kb
jpg - medium ------ 8378 kb
So I can make the PDF version for distribution only with JPG compression
This is not optimal for graphics.
I think the methods for "automatic" should be worked over.
All what's imported from a PNG should be compressed looseles.
All what's imported as a JPG should be compressed loosey.
Sometime's, it happens that a PNG graphic shrinked to a smaller size has more kb than the original.
So the decision should be:
Shrink graphic to dpi
Compare size with original
Use original graphic if size is smaller or equal
I guess the major question here is: What image compressions are allowed in a PDF?
I guess this page gives a hint on what formats are possible to use.
http://en.wikipedia.org/wiki/Portable_Document_Format#Raster_images
Scribus main target is printing, when printing the print quality is more important than file size, that's probably why the automatic setting gives big files.