Pixels are the actual dots that create the text or images.  Within each pixel are the actual lights or diodes called LED's (Light Emitting Diodes). 

Pixel Matrix is the arrangement, or array, of the pixels (Ex: 16 x 64). 
16 pixels tall (or rows) by 64 pixels wide (or columns).

Pitch is the distance between pixels.  Some companies use the term °resolution±.
Most small LED displays have one LED per pixel and large LED displays can have anywhere from 4 to 25 LED's per pixel.

Pitch determines the size of the text. A 17mm pitch displays 5± text as compared to a 34mm pitch which displays 9± text.

As the distance between pixels increases (the pitch), the more LED's per pixel is typically required, so the text is fuller in appearance.
Typically, a wider pitch is required for LED Displays that need to be read from a long distance.

Viewing Angle, typically means, how far the user can walk from center facing the LED display in either direction and still see the same level of brightness .  Most companies refer to viewing angle as readability and not brightness. 

There are two common types of LED's on the market: a 30 degree diode and a 70 degree diode.  This degree refers to the brightness.

On a 70 degree diode, pass 35 degrees from center, the brightness will fade a bit, but the LED Display is still readable up to about 70 degrees from center, creating a 140 degree °readable viewing angle±. On a 30 degree diode, pass 15 degrees from center, the brightness will fade a bit, but the LED Display is still readable up to about 45 degrees from center, creating a 90 degree °readable viewing angle±.