Default Application Issue

i want to be asked everytime if i want to use an application 'always' or 'just once'. but that never happens.. After i reset my default applications for certain file types, and choose an application to open it, that application becomes the default application without asking my permission.

For eg. I like to open multiple PDFs at once and switch between them quickly. and that is only possible if I open the different PDFs using different apps. But once I use Adobe to open a file, all PDF files open using Adobe and I am never asked later on if I would like to use another app to open a PDF.
(I have not chosen the 'Use Always' option. In fact, i am never given that option or 'just once'. all i can choose the first time after reseting default apps setting is the app i want to use to open the file.)

i have attached herewith an image to show you what is shown when i open a pdf file. (i am asked this only the first time and never after that)

Please help.