Thanks for all the responses. I tried to save as postscript instead of pdf before converting to vector graphics and the result is exactly the same. I have to convert the postscript to pdf before converting to vector graphics, so that might have something to do with it. On the other hand, if bits that are not visible had been removed when the file was saved as postscript in the first place then they should not suddenly pop up. As far as I understand the responses it looks as if that is a built-in problem of postscript/pdf? Or can that be fixed somehow. It is obviously not a very high priority problem, just a niggle.