Layers and exporting pdf

[steelypan]

Dear users

I wonder if you can help? I created a complex illustration in Designer Pro 6. I only have 1 layer in layer gallery, but because of transparencies etc, items are moved forwards and backwards to create depth. I always 'select all' and group items before exporting for print. When I then export the design as a 'high quality' pdf, it appears the pdf is still creating 'layers.' An online printing company has told me to 'flatten' pdf, to avoid problems with uploading the file. Is there something I'm missing? If I export the picture as a bitmap, then re-export as a pdf, will I lose quality of image. Is there a way of combining all the elements onto a single layer?

Thanks

Steelypan