Screws for fitting to PVC windows?

I would go for some of the uPVC repair screws, they have a wide thread.

Won't be solid, more likely a box profile of some sort.

Depending on how deep the frames are, assuming they are hollow you might be able to use small spring toggles as are usually used for plasterboard. With plasterboard you can get away with pushing through the hole but with PVC you'll need to drill an oversize hole to accommodate the closed toggle.

The screw would probably need to be cut down to size

The principle is explained on here -

michael adams

I've always used x6 chipboard screws. Never had a failure yet. Drill a pilot hole (2mm?) and the thread cuts nicely into the pvc

If your are mating two flat surfaces and they have a reasonable area where they are in common contact with each other I would consider using one of the self adhesive backed push together fastenings . Hook and Loop tape is the most common variety but there are stronger types such as 3M dual lock. As ever performance depends on what quality is used and where a genuine Velcro or 3M product will be successful a cheap copy of ebay may not suffice. A decent version will be more costly than a few screws but you won't be putting holes into a thin walled frame which may not prove strong enough over a period and should the blind go out of favour the tapes could be removed using a suitable adhesive solvent and no unsightly holes left.

G.Harman

Same here.