I have a lot of PTO to take, which will make the deadline impossible

The clearly correct solution here is for your employer to give you a one-time waiver and allow you to carry over some of your leave to next year. That makes the client happy because you get the job done and you happy because you don't lose your leave.

Just make sure you spread your leave out in the year next year so you don't have to have this conversation again next October.

Starting with the premise that everyone is replaceable. Doesn't your firm have a plan if you were hit by a bus (sort of joking) or left. The project should be able to move forward without you. Maybe not as quickly, but still move forward.

With this logic as a starting point and the fact that you are going to take PTO, along with a hard deadline then delegate some tasks.

Can't or won't your leadership provide you some help for simple tasks to help free up time?

Since other projects are planned in the future, it is a safe bet for your company IMHO.

Note that even if PTO can't be carried over, a manager can usually do so unofficially by calling it flex time. Management will frown if that's done too often, since the point of vacation from their point of view is that you should take time off to recover now and then, so you probably want to make a greater effort next year to plan your vacations and burn off everything you are required to.

In fact, planning ahead is the key to handling vacations. If they know in advance when you will be gone, your company and the customer can work around that, even if you are nominally essential; they will just figure it into their schedules, and/or find someone else to put out fires for that week.

Note that if you are absolutely essential, it is long past time for you to train someone else to be able to handle these tasks. Management should allow you time to do so. If they object, remind them that if you get hit by a bus and there's nobody else ready to step in, it's going to be far more expensive to recover. ("Bus factor" is a legitimate management term.) Being essential is less valuable to the company than being able to teach. Among other things, being able to teach frees you and them to consider moving you to new assignments.

You have left out the rather important detail of how much PTO you want to take.

Typically health and safety legislation will require a minimum level of PTO and often it is a requirement that staff take this. I'm actually getting pressure from my own employer to take all my PTO by year end for this exact reason. Check your local laws on this subject.

You have PTO to take and it's not in any sense unreasonable to want to (and need to, for personal sanity) take that PTO.

• It is entirely up to the company you are employed by to allow for this. It is not in any sense reasonable to try and force or pressure you to not take PTO.
• It's the company's job to provide alternative cover for the client if the client wants this.
• It is entirely up to the company that employs you to square this with the client - it is not your problem.
• Email your manager of your intent to take either all your PTO, ideally giving dates, and say if they need you to work some of that time they will need to say what dates they need you for and what alternative to using all your PTO they can offer.
• Explicitly mention that your company's manager will need to communicate the need to take this PTO with the client.
• If they want you to sacrifice PTO for no compensation, just refuse.
• If they want you to work more PTO (with compensation you're happy with) than your minimum personal need would allow, then raise this issue. You can negotiate in any reasonable way you wish.

You have two obvious choices. Don’t take your PTO, lose out completely, your client is happy and you’ll be very lucky to even get a thank you. Or take your PTO, and your client is very unhappy. Both choices are very bad for someone.

The solution is to discuss this with someone high enough to make a decision, so the client gets his work done, and you get compensation for not taking your PTO. The employer can also do other things to help. I worked in an office 100 meters from a very nice hotel. Pay for me to stay there and get my clothes cleaned etc. and I can work 60 hours a week because I have no travel, no other work and so on.

The deadline of the project is already too ambitious (in my opinion), even without factoring in my PTO days. I want to take all of my PTO. How can I communicate this clearly without shooting myself in the foot? The contract with this client runs for roughly another 10 months. The days are not yet planned, and to be honest, I don’t have any concrete plans yet.

With 10 months to go before the deadline, take the days. Working without taking a long weekend or even a full week off will burn you out, which will make the deadline even harder to achieve.

If you said I need to take x days of leave in the next few weeks, but you can't because that period of time includes an critical deadline, then the issue would have been lack of planning.

There is time to plan your leave. Everybody needs to take the leave they are entitled to.

---New information from the updated question.---

The deadline of this specific project is in 3 months (end of December). My PTO has to be taken by then also.

With about 13 weeks to take 19 days, you may have to sacrifice some of your leave. But you can't sacrifice all of it. You mention 28 days per year, which means you can be expected to be on leave about 2 days a month. They should have zero problem with 6 or 7 days in 3 months, if they are well timed to not fall on a critical day. You can even stretch it to 10 or so, without too much objection.

When you have a large number of days to take each tear, and you have a series of projects/customers you need to be mindful how the days hit the customers schedule. You need to take long breaks between projects and short breaks while working on a project. Customers don't like it when a key person is gone for a significant portion of the project time.