Where can I find the U-factor (U-value) for a Pella window?
As with most items, there are good, better and best. With Pelli, you can break that down into another hundred parts or so. 

Background:

In the construction industry, most items (locks, carpet, windows, roofing, etc.) are divided into “design grade” and “spec grade”. 

Design Grade is the top grade and used by architects and contractors for top performance. Here clients are looking for the best long term performance. Here’s a sample of what architects select from: https://content.pella.com/cs/groups/public/documents/document/mhat/mdy2/~edisp/p-066812.pdf?_ga=2.190888361.2053217315.1575606363-1340307589.1575606363

Spec Grade is the lower grade and is usually used by spec builders, box stores, etc. These products are often chosen based on lowest cost. (If a manufacturer can “save a buck” to get the sale, they’ll do it at the expense of performance.) So a box store often has one low cost (and low performance) for home owners to choose. 

Summary:

The “U value” of windows depends on 1) quality of frame, 2) number of panes of glass, 3) argon gas, 4) low E coatings, 5) air space between panes, etc. 

So, in your “Example 1” where they show the window has a U-value of 0.27 it’s because the are using their lowest quality (and least expensive) frame and glazing. 

However, in your “Example 2” they do not list a U-value because there are a gazillion glazing and frame options. (Thank you Pella for making it so complicated.) Here’s a sample of some U-values (appears to go down to 0.15) for a couple different style of Pella windows: 
www.pella.com/professional/adm/Clad-Wood/Pella-ArchtctSrsTrad_Awning.pdf

Take your pick, but it looks like you can get something significantly better than the “Spec” windows at a box store...if you’re willing to pay for it. 

Fortunately, Pella is a high service manufacturer, so call them and they’ll walk you through the performance of the options. Then call a glazing company to get a bid.