What the consulate is checking
A short-stay (Type C) Schengen visa requires evidence that you’ll leave before it expires. A confirmed reservation showing your entry and exit dates demonstrates exactly that — which is why a held PNR is the sensible middle ground between nothing and a fully-paid ticket.
The 90/180 short-stay rule
Schengen short stays are capped at 90 days within any rolling 180-day period. Reservation dates that imply a longer stay will raise immediate questions, so keep them inside the limit.
Don’t forget accommodation and insurance
For tourist and visitor applications you’ll usually need a confirmed hotel reservation for your whole stay, plus travel insurance with at least €30,000 of medical cover valid across all member states.