I have created the PCB design to build large 7 segment display based on WS2812 leds (design attached). But now I am looking at the datasheet and I see capacitors for individual leds.
The question is this PCB as is would work if I add one capacitor between the + and - pins powering that board.

 

It might sometimes.

It might not sometimes.

A single cap per digit probably isn't enough. The datasheet recommends one per LED, and they wouldn't have made that recommendation if it wasn't needed.

 

WS2813B does not require caps.

Pedantic nit: don't connect your SMT pads at angles other than 0, 90, 180, or 270°.

Otherwise, someday this will bite you when your software automatically adds thermals.

 

It might sometimes.

It might not sometimes.

A single cap per digit probably isn't enough. The datasheet recommends one per LED, and they wouldn't have made that recommendation if it wasn't needed.

Yes, it depends.

For example
The Chinese 1.3 version of the WS2812B-2020 datasheet says you don't need them. That version seems to have on chip module decoupling. NOTE WS2812B-2020
https://jlcpcb.com/partdetail/Worldsemi-WS2812B2020/C965555

Found a english version.
https://www.tme.com/Document/8b264124130de6e17f8ce807c3210601/WS2812B-2020.pdf

 

If what we are shown is the LED side of the PCB and the circuit boards already are made and the pars attached, it can still work to install at least one 0.1 mfd/ 5 volt capacitor between each pair of devices. NOT SMD capacitors but wire lead caps with the leads soldered to the traces. I have done patches like that and they can work well.