yes and no
What printing machine do they use?
Inkjet or something more serious?
I talked about a raster image processor, which is used to do the big style printing (offset and so on)
The problem is that a normal printing machine can only do four colours:
It cannot print let’s say 50%black. So what it does is putting larger points together by using many of its smallest availabe points.
example (not realistic, but more clear):
You want to print with 150 lines per inch. That means 150x150 points per square inch.
If the printer had a max resolution of 150 dpi, it could only print cyan or cyan and yellow or such combinations by printing the rather huge dots over eachother. not much of a colour variety.
lets say we double the printer’s dpi. So it can print 2x2 dots of every colour in one of your points. Now the printer can choose to set one or two or three or four dots in the area that one of our points. So it could choose to fill one point with half magenta and half yellow. Much more freedom.
If you have a ratio of 10 to 1, you have 10x10 dots for one point and you can choose theoretically from 0 to 100.
Really good quality is a ratio of 20 to 1, so you have 400 dots to assemble one point of your printed work.
But why can one not say 1 pixel of the picture for one point in the printed work?
Because the grids of the four colours are not horizontally aligned.
One version is:
C 75 degree
m 15 degree
Y 0 degree
K 45 degree
So the more pixels it can use to build the printing grid out of, the better.
So 10 times more ppi than lpi and 20 times more dpi than lpi is good.
You can see what I mean if you look at a cd cover with a magnifying glass or a binocular turned upside down or a good webcam or a good scanner
But if your customer wants it to be 300 dpi and 20x30 inch, Stefano is quite correct.
The problem is two things.
cmyk and ppi.
As far as I recall, you cannot change the dpi in blender. And blender can only give you RGB, suitable for screen.
So save as targa (NOT JPG)
Go into an image processor (photoshop is best for print)
change the format to cmyk
and make sure the ppi of the image is set to 300 without changing the amount of pixels.
and save as TIF. EPS is even better, but a bit tricky for its versions.