Most of the tips given will not help much, because the main reason that Scribus PDF becomes big is NOT bitmap images.
The reason Scribus PDFs become big is that Scribus is very detailed about the placement of objects, including text. If you create a PDF from a word processing program it will most likely print the text line by line (i.e. it will set coordinates and then print a whole line of text). As I understand it Scribus places text glyph by glyph. For each character on a line it will set coordiantes and then print the glyph. For a text with 80 characters per line this more or less makes the file 80 times bigger (because instead of just placing each line Scribus will place 80 characters per line). The devs may correct me if I'm wrong, maybe this is something that has improved in later versions? /Peter
