I really hate being the language police, and my own mastery isn't all that great, but....
I want to apply for a company as a PA...
No, you want to apply for a PA job with a company (or at a company). Or... You want to apply to a company for a job as a PA.
... but not sure how the PA's schedule works if their working for a company.
... how the PAs schedule works if
they're working for a company.
Clear and concise communication!!! That's what filmmaking is all about, and very important in your business life. My language may not be elegant and is far from perfect, but at least I express my ideas clearly.
Now, on to your question...
You can work as a freelancer or as a member of a staff.
As a freelance you will be project based. You will have to hustle every day you are not working, and even more when you are working, to get more freelance jobs.
When you work for a company, even though it's the entertainment business, to a large degree it's still a corporate job. The hours are fairly regular, but you will most probably be "asked" to do overtime; if you are on salary you may not be compensated for the OT, but you dare not refuse if you want to get anywhere. The pay is regular and you may get some benefits, but you are a cog in a machine. You'll have to work very hard and have to osculate a lot of posteriors to be noticed.
The "secret" to moving up is to do every disagreeable job with a smile, always ask for clarification if you need it, never ask the same stupid question twice, and develop a reputation for punctuality, hard work, agreeability and reliability. Appropriate dress and good personal hygiene count for
A LOT. Even then you need a little luck.