Does Riven's Passive increase if you increase your attack damage? I know it increases with level but I have noticed that if i start with a doran's blade the passive is stronger.
It adds a percentage of your total damage to your attacks. So yes it increases with your level. Since doran's blade adds "flat" damage it's obvious doran's blade makes the ability stronger. Any item that adds attack damage will increases the power of the passive.
However, early on this is much more effective. Let's say you do 100 damage, a 50% bonus would mean 50 extra damage. Getting 50 extra base damage would give you 75 extra damage, a 150% increase. Adding another 50 damage would give you 100 extra damage, an increase of 133% extra damage.
It does scale with attack damage.
Riven's abilities charge her blade for 5 seconds, causing her to do 20 / 25 / 30 / 35 / 40 / 45 / 50 % of her attack damage as bonus physical damage on her next basic attack. Riven can store up to 3 charges, and can only expend one at a time. (Source)
The percentage value changes every three levels, so it's 20% at level 1, 25% at level 3, 30% at level 6, etc.