Image quality

[rich2005]

Quote:Does GIMP actually change the rasters into vectors?

I have never found that Gimp converts rasters to vectors. Giving Gimp the best chance in the world, simple shapes each on its own layer. A small difference in file size. (1) Usually, single layer, no difference.  As a comparison a PDF from Inkscape (vector editor) same size / layout  is 1.2 K vs. Gimp 60 K

   

Quote:And why does the file size go from 4M to 300K  (.tif to .pdf) when PS maintains the same file size?

Tiff format comes in various forms, uncompressed, lossless compressed.. file size can vary considerably. 
The example shown, all the same image, (2) jpeg 400K / (3) compressed tiff 4.5M / (4) uncompressed tiff 10.5M  Once opened in Gimp there is no format just pixels.
Exported as a PDF the only real difference is jpeg v. tiff but in all cases the PDF is not tiny.  There will be some compression by Gimp (probably RLE). 
Just a guess, I think that PS is just 'bundling' up whatever the file is, large file size or small in a PDF 'wrapper' . You can get that with say, ImageMagick 

No idea why your tiff file should shrink so dramatically, does it contain large expanse of solid colour that compresses a lot?  Are the tiffs 16 bit images down to 8 bit in the PDF. Too many unknowns.

edit: Just a thought. A tif file can contain a thumbnail image. That would show opening in Gimp as two pages, make sure you use the correct one (page 2 ?). It should be obvious from the image size shown top of the Gimp window.