Do you have any ideas for improving the image analysis for bubble shape ?
Cezary,
I guess the basic problem is the really low resolution of your image:
ImageDimensions[img0 = First@Import["original.pdf", "EmbeddedImages"][1]]
(* Out: {17,256} *)
BTW: A probably equivalent result can be achieved somewhat more simple like so:
FillingTransform@ColorNegate@Binarize[img0]
