Pros
It's the biggest CRO, there's job stability, and everyone knows PPD. Once you have monitoring experience, you can go anywhere else.
Cons
The pay is almost never competitive with other CROs. Management is constantly changing expectations with no transparency and little to no time to adjust to new expectations without it negatively affecting your reviews. Your work-life balance depends entirely on the studies you are assigned to - if you are over-allocated or assigned to an unreasonably demanding study, there is no help for you to accomplish the extra work expected and your personal life will suffer for it. You may not get a manager who supports you and tries to help; if you don't get a supportive manager, expect to drown in work and get bad reviews for it.