The main problem here seems to be the understanding of the difference between raster or pixel graphics on the one hand and vector or line graphics on the other hand. The difference is explained in the questions already linked by the OP or in the Wikipedia article on vector graphics.
Typical file formats for raster graphics are
.tiff (read more here).
Programs for creating & editing raster graphics are Microsoft Paint, Photoshop or Gimp and many more.
Typical file formats for vector graphics are
.svg (read more here).
Programs for creating & editing vector graphics are Inkscape, Corel Draw, Adobe Illustrator and many more.
Most vector file formats can also contain raster graphics, e.g. as background decoration. Including or converting a
.png file into a
.pdf file does not make it a vector graphic.
On the other hand, converting a vector graphic into a raster graphic makes it a raster graphic forever, there is no way back (apart from vectorisation, which is usually as laborious as creating a new graphic from scratch).
gnuplot and also Matlab, Octave, matplotlib etc are capable of exporting vector graphics:
eps if your workflow is
pdf if your workflow is
.wmf iy you are using Microsoft Word.
The OP also asks about block diagrams created with MS PowerPoint. PowerPoint internaly uses vector graphics for block diagrams but does not allow export to vector graphics directly, not even to the vector formats developed by Microsoft (called
.emf). The workaround here is to export to pdf, either by "printing" to a pdf printer like Adobe Acrobat Distiller or pdfCreator, or by using the "save to pdf" function which is included in MS Office 2007 and newer. This approach also works for Excel diagrams.
As an alternative way to create plots and block diagrams, I would suggest to look at TikZ/PGF.