what does it means this advice? Object is a placed PDF , i inserted a pdf logo 72x72 dpi
This is mainly a warning and as far as I remember it relates to two issues:
1. Normally Scribus will rasterize a placed PDF during PDF export (there is however a function labeled as "experimental" that will include the PDF "as is" at export).
2. For a placed PDF Scribus will not apply color management.