again thanks for your detailed and patient explanations giving me deeper insight into various aspekts.
So I took several hours to work it all out.
As result came out: only Adope Reader will show the images in the same dimensions as the originals after I set '72 points for inch' which was unchecked before. (Changing the units didn't make a difference). With this reader the pictures show off very sharp and brilliant.
The other Pdf-Readers I use, don't have the option to set the points for inches and they will continue to display the pictures double-sized if the zoom is at 100%. (Useing different picture-sources or changing the units had no influence on that.) Here I can choose something around 66-70% to get near to original largeness, but never getting a sharp display.
On an inspiration I opened the pdfs under Unix (while surfing) and both of the onbord readers displayed them correctly straight away.
To try a conclusion I'd say it depence on the capability of the reader to interpret the pdfs the wanted way. Further dependences concering the local settings of an OS is far beyond my knowledge but presumingly another source of reason why this effect occurs.
So - if nobody else but me meets this behaviour there shouldn't be further bothering about it, but still I hope it was good for something (at least I learned a bit from it

nice regards ~ Vera