Puppy is 5 months old.
Have been using PPP. Pup growing fast and sometimes stiff getting up in the am.
Was considering switching to PP Lrg Brd Pup but there doesn't seem to be much difference in ingredients
The protein is only slightly lower (from 30% to 28%) and the fat is quite lower (from 20% to 13%).
My pup is growing beautifully. Has tons of bone, body, good weight, looks wonderful. I am afraid that switching to PP Lrg Brd Pup will ruin this. Pup is not fat so I do not want to lose weight.
What should I do because I am also scared that if I keep using PPP the pup is going to end up growing too fast and damaging ortho.

Get the dog off the high test and onto adult.

Purina has some great articles and studies on their Purina Pro Club site. But one also has to remember that each dog is going to be different.
I have had great success with feeding large breed puppy formulas. Never had a case of pano, etc. My first "large breed" puppy I ever feed and raised on an adult food was a disaster!
Nearly 10 years of feeding LB puppy foodsFed until 9 months of age. Nice even growth rates.

We always switch to a regular formula adult at around 4-5 mos of age. Just what seems to work for us here.

The only problem with this is that I don't use Pro Plan adult due to it being impossible to get the regular formula. All we have access to is shreds and I refuse to feed that garbage.
My adults get PPP, which actually is an "all life stage" formula BTW.
I guess I'm just having issues because I'm second guessing the high protein/fat.
Does anyone have an opinion as to whether it's the fat or the protein that can cause fast growth? Which could be harmful if fed too much of?
I have heard conflicting stories.
I'm about to switch this pup to Exceed L&R and be done with it. But I also have concerns over this because someone once mentioned it wasn't good to raise growing pups on Lamb.

Actually Foodie...this is what I switch my growing pups to...Exceed L&R. My adults get Exceed C&R but I worried that it was too high protein/fat for babies. I have done this for several years now with good results. You are welcome to contact me privately if you wish...I have provided my email.

If a 5 month old puppy is stiff getting up in the morning, I wouldn't worry about food first. I'd have him in for an exam and a tick borne disease test.

That being said, you should be able to get Pro Plan Salmon (Sensitive Skin and Stomach) which is a "regular" version of Pro Plan. I wouldn't feed Large Breed - I feel it's a consumer-oriented gimmick food. If a puppy is growing too fast, feed it less of the highest quality food. At this age for me it's a toss up whether I feed Puppy or Performance. I feed Performance to all of my dogs of all ages, except puppies up to 4-6 months and lactating moms. On occasion I throw in a bag of "regular" which these days has only been Salmon to vary the protein source.

The science behind orthopedic health is basically keeping a young dog lean will significantly prevent a lot of orthopedic problems. You could feed anything. The study at Cornell was done with Purina Dog Chow. I feed the high protein because it gives a better coat. Haven't found any difference in varying fat levels as long as the fat soluble vitamins are there.