Comment 7 for bug 1846464

Here is a hypothesis for what is going on: It's working,
doing what it was told to do, but what it was told to do
is probably not what is intended.

First, a closer look at the example input PDF shows there
is an oddity (not wrong, just odd):

   $ pdfinfo -box Micro-Magic_Plan_2016_V2.pdf
  [...]
   Page size: 2961.3 x 2094.21 pts
  [...]

However, A4 paper is 210 x 297 millimetres, or more to the
point here (excuse the pun), 595 x 842 pts. That means the
full-sized image is already more than four times as large as
a single sheet of A4 paper (portrait, which is the default).
Hence, `--scale=4' is going to produce an image that is more
than 16x the size of A4 width (and correspondingly as high):

   $ pdfposter --dry-run --verbose --verbose --verbose --scale=4 Micro-Magic_Plan_2016_V2.pdf /dev/null
   Mediasize : 1.0x1.0 a4
            595.00 842.00 dots
   Scaling by: 4.000000
   ---- processing page 1 -----
   input dimensions: 2961.30 2094.21 (trimbox of input page)
   output dimensions: 11845.20 8376.84 (calculated)
   Pages w/o rotation 20 x 10
   Pages w/ rotation 15 x 15
   Decided for rotation: no
   Deciding for 20 column and 10 row of portrait pages.
   ^C
 [...]

That's a BIG poster: 20 A4 pages wide (4.2 metres), and 10 A4
pages high (almost 3 metres), on a total of 200 sheets. I'm
not surprised it's taking a long time to generate a very large
output. (The use of `--dry-run' and `/dev/null' are attempts
to avoiding having to ^C (unsuccessfully).)

Presuming what is wanted is a poster four times the size of a
sheet of A4, then, since the image already is about that size,
my suggestion is to use `--scale=1':

  $ pdfposter -m2x1a4 --verbose --verbose --verbose --scale=1 Micro-Magic_Plan_2016_V2.pdf Plan1-landscape.pdf
   Mediasize : 2.0x1.0 a4
               1190.00 842.00 dots
   Scaling by: 1.000000
   ---- processing page 1 -----
   input dimensions: 2961.30 2094.21 (trimbox of input page)
   output dimensions: 2961.30 2094.21 (calculated)
   Pages w/o rotation 3 x 3
   Pages w/ rotation 4 x 2
   Decided for rotation: yes
   Deciding for 4 column and 2 row of landscape pages.
   Creating page with offset: 0.00 1190.00
   Creating page with offset: 0.00 0.00
   Creating page with offset: 842.00 1190.00
   Creating page with offset: 842.00 0.00
   Creating page with offset: 1684.00 1190.00
   Creating page with offset: 1684.00 0.00
   Creating page with offset: 2526.00 1190.00
   Creating page with offset: 2526.00 0.00
   $

That's a full-size image on only 8 sheets (landscape),
or just over one metre wide and sightly less than 500cm
high (half a metre). Which seems, to me, reasonable.
(I do NOT know if the `-m2x1a4' is actually useful?)

And it does work, albeit it still takes a long time to
process: On my system, 12 minutes (100% CPU), generating
an 80 MiB output file. (I have not printed the output,
but it looks Ok in a PDF viewer (Okular).)

Scaling up to the original 200 sheets of `--scale=4'
suggests it would take about 300 minutes (5 hours),
and generate a 2000 MiB (2 GiB) output file. I do not
intend to test these estimates! ;-)