Pianoteq 5 on Linux, howto install/start ?

The file without extension is an executable (Don't think strictly in terms of file extensions with linux -- They are sometimes used for convenience but don't have any essential function. There is no accepted standard file extension for executables).
The .so file is a library, like a dll in windows, and should be kept in the same directory as the executable.

You should have executable rights to the file after unpacking; Otherwise, you can add them with chmod +x (As I'm sure an Arch user would know..)

Have you experienced music with Linux before? You must configure and open Jack before (+/- equal to Asio but on Linux) . Not difficult but it is a different philosophy than the windows world.
To just answer to your question: There is a simple executable Pianoteq file. You have nothing to install. Just click on it, and it will start. But start Jack before.
There is also a Pianoteq LV2 folder. LV2 is the current standard plugin format on Linux. To use Pianoteq as a plugin, you should copy the Pianoteq LV2 folder in your standard LV2 folder, which is probably "usr/lib/lv2".

If you want to do music on linux, I recommand you to try Tango Studio, a distribution specific for music, with real time kernel.

Good luck

Jack is only necessary to run Pianoteq in conjuncion with other audio applications. If one just wishes to play, using ALSA alone is easier (and I think faster, latency-wise, since even with Jack, audio will be routed through ALSA anyway AFAIK).
It is in this way that I usually use Pianoteq.  Much less hassle.
One has to make sure pulseaudio is not running, though, which is easily achieved with pasuspender.

ALSA is the hardware driver infrastructure (i.e., backend), while JACK is an audio server, akind in some ways to the Windows Sound Mapper. JACK allows network-transparent sound input/output and (more importantly) allows audio applications to work together. You can achieve some results this way (by piping sound output from one application to another) that would require loading a plugin in a DAW on Windows or Mac OS. But if you 'just' want to use Pianoteq as a live instrument, you will not gain much by using JACK, and while it is designed with low latency in mind, it is an additional component in the audio chain. That said, you will not lose much as well by just always using JACK, and gain flexibility.

Edit: PulseAudio is also an audio server, but as far as I know, its design goals are quite different from those of JACK, meaning more functionality and quite possibly a lot more latency. So if PulseAudio is the default on the system you use, I would bypass it if I were you, or switch to JACK.

A bit background on Unix-like systems and audio: nowadays you will find at least three, often four audio interface layers on a modern system. E.g. KDE4 uses Phonon as multimedia framework. Phonon knows how to decode audio files and has functions to 'just play an MP3' or such file. This is used by normal KDE applications that just want to play sounds and not care too much about the details (which is sufficient in 99% of the cases, probably). Phonon can be configured to use different implementations, one of which is GStreamer. GStreamer does the heavy lifting and can e.g. be configured to use JACK or PulseAudio (or ALSA directly). JACK again will most often use ALSA for output. So you have a lot of layers:

1. Phonon/...

2. GStreamer/Xine/...

3. JACK/PulseAudio/...

4. ALSA/OSS

An application can use each of those to output audio. The lower it goes, the more work it will have to do for itself, but also the less latency and more control it will get. Pianoteq is an application that does by definition most of its own low-level work, so only the lower layers really make sense for it.

Unfortunately, you're out of luck.  Pianoteq does not run on ARM.

Ok, it's theoretically possible to run a x86 emulator/VM under ARM to then run PTQ but good luck w/ performance.

Probably none of the above, sorry. I don't even know whether an ARM based VST host exists; but even if it did, it wouldn't matter. ARM VSTs would in theory be possible, but this does not change the fact that the processor architecture of the Pianoteq VST is x86, and thus the architectures don't match.

The requirements of Pianoteq are unfortunately hidden in the FAQ, but they are listed:

The architecture limitation goes for Wine as well: it is an implementation of the Windows API, not a classical emulator, and thus will not enable you to run x86 code on an ARM CPU. Emulating CPUs is so complicated that emulation will cost you about an order of magnitude in terms of performance... and the Exynos would be barely fast enough as it is.

So your only option as far as I can see is: get an x86-based computer, sorry. I would also recommend aiming for a PassMark score (http://www.cpubenchmark.net/) of at least about 1500-2000 in order to get a polyphony of 64 notes or more. Pianoteq's requirements are quite moderate, but they are non-zero.

PS: if the Mac OS X 10.5 requirement is still accurate, Pianoteq could actually still be able to run on PowerPC computers, which is a big-endian architecture like ARM. This in turn indicates that it would probably not be actually impossible to port Pianoteq to ARM, but I think Modartt will only do this when (1) the performance of ARM devices increases significantly (they are quite efficient and relatively powerful for their low power consumption levels, but not on an absolute scale) and (2) actual devices are available and (3) there is actual demand. And I would guess that the third point is the most tricky.