I'm not sure whether there could be a singe PDF containing all the problems over the years (because it would be pointless?). Most contain limited number of problems of one topic (e.g. Inequalities). There are books, however, which contain most, if not all, of the problems over the years, with multiple solutions and theorems and methods and techniques and what-nots. If you are planning on doing really well on the IMO, then don't only concentrate on the official six problems. It is neither ideal nor good enough for practice purposes. Do the longlists and shortlists (a warning, though - some shortlisted problems are ridiculously harder than those which make it in the end, which might as well explain why they were omitted to begin with).