Unless there are objects filled with white this is perfectly normal.
how can i export only white objects to pdf
Can you clarify: are as you say "images" (e.g. PNG) or are they vector.
Do you have an errant transparency effect applied to the white objects that might be affecting how they interact?
Can you upload a sample file?
White is often treated as the paper color. You may need to create a custom spot swatch and of course definitely check the artwork in AI beforehand. Also your artwork looks like it is using a knockout group, so you may need to check the appearance panel for how that stuff is arranged and may need to reorder and convert it.