How to convert color eps to grayscale

That turned out more complicated that it seems. The default ways didn't seem to work for me, because the eps was produced by Maple (so there is a bitmap graphics inside).

Here is what it worked for my figure.eps:

$epstopdf figure.eps
$gs  -sOutputFile=output.pdf  -sDEVICE=pdfwrite  -sColorConversionStrategy=Gray  -dProcessColorModel=/DeviceGray  -dCompatibilityLevel=1.4  -dNOPAUSE  -dBATCH  figure.pdf
$ pdftops -eps output.pdf

This can be done easily to a script like this:

#!/bin/bash
epstopdf $@ -o output_eps.pdf
gs -sOutputFile=output.pdf -sDEVICE=pdfwrite -sColorConversionStrategy=Gray -dProcessColorModel=/DeviceGray -dCompatibiltyLevel=1.4 -dNOPAUSE -dBATCH output_eps.pdf
pdftops -eps output.pdf

Save it as epstograyscale.sh
$sudo chmod +x  epstograyscale.sh
$sh epstograyscale.sh figure.eps

It worked on my PC.

How to convert png to pdf

Ok, as always Libre Office Impress does strange things with slides design when I do export to PDF so I got creative.
1. Export to PNG from File->Export (you could do it for all slides trough Export->Html, but somehow the quality appears worst)
2. Make sure you name you pngs correctly: Pic001.png, Pic002.png
3.  convert Pic0*.png -quality 100 document.pdf
Et voila!
Now, convert has some pretty cool options, make sure to check the man pages if you want to do something more fancy.
Another cool thing I discovered is how to convert png to eps!

sam2p pic1.png EPS:pic1.eps (source)
It works awesomely and makes even 3d plots look good. Of course, you could use also convert:

convert fig.png eps3:fig.eps 

(source)
Nice huh?