Learning a new skill is fun, especially one that could land you a new job. We don’t think you’ll find too much demand for PDP-11 assembly language programmers, but if it still interests you, check out [ChibiAkumas’s] video that starts a series on that subject for “absolute beginners.”
The PDP-11 is a venerable computer, but you can still find simulators ranging from SIMH to browser-based virtual devices with front panels. If you want real hardware, there is a PDP-11 on a chip that is still around (or you can score the real chips, sometimes) and there are some nice hardware simulations, too.
Like a lot of machines in its day, the PDP-11 was most comfortable with octal or base 8. That’s a bit odd these days, but it is easy enough to figure out to express for example 377 instead of FF.
The text part of the tutorial is a little — um — colorful. A “reading view” like the one you get from Mercury reader or in some browsers can help if it bothers you. You might also try PrintFriendly.
If you are into specs, a 1970-era PDP-11/20 ran with a cycle time of 1.5 µs and 56KB of magnetic core. All for the low, low price of $20,000 (although that probably didn’t include the TeleType machine or any fancy I/O such as tape drives).
Given the limited amount of memory and the simple instructions, it is amazing how much these old computers can do. Even today there is at least one nuclear power plant that uses a PDP-11 to control some robotics. In Soviet Russia, several home computers used clones of the LSI-11 which was a PDP-11 on a chip. The Heathkit H11 used the same CPU.