I see people recommending items without giving reasons, and that makes me sad.
Varus can do many things other adc's can't. He can poke, trade and farm very efficiently pre-6 and can lock down several people past-6 while doing good dps. He has %-magic-dmg as well as high spell base dmg on E and high spell ratio on Q. His main advantage in a trade with other adc's is his E which reduces healing by 50% and slows, as well as his W giving him magic dmg on-hit and %-magic-burst.
He lacks reliable attack speed and gap closer in his kit.
Buy BT or BotRK for trading, Infinity to scale lategame, Stattik Shiv to push, move faster and deal massive burst midgame, Last Whisper for ArPen to benefit his high skill damage and Guardian Angel / QSS / Randuins to prevent Burst / Lockdown / Dive.
Earlygame Dorans help to comeback or dominate, but are a high risk-reward buy.
So Shiv is better than PD?