Image quality

[rich2005]

Quote:...But, when I try to save it as a .pdf file, the file size becomes approx. 312K !
How did my friend save it in Photoshop as a 4M .pdf file, and how can I save it in GIMP as a 4M .pdf file?

Too late, go back and start again.

A PDF from PS might contain raster images (images made up of pixels) as well as embedded text and maybe vector objects (arrows, stars..etc)  These open in a PDF viewer and can be viewed in good quality from small to large size.

Gimp is a raster editor. Open the PDF in Gimp and everything is converted to a single image made up of pixels.  Text is also converted to raster, this does result in loss of quality when viewed at high 'zoom'

By default Gimp will import the PDF with a resolution of 100 pixels-per-inch (ppi). Not only Gimp other applications do the same.

This has a big influence on quality. Assuming  your PS PDF contains a photographic quality image then import the PDF with a ppi of 300.  Settings like this:

   

If you are using Gimp 2.10.22 and added text, it seems to add the text metrics correctly. Earlier 2.10  versions, not so good, flatten the text before exporting.  
Export settings ? For a single page it does not make much difference. I leave everything checked. I have never seen Gimp convert a bitmap to a vector.   Gimp 2.10 Multi-Page is a disaster, text layers export to separate layers, avoid.  
File size ? All depends what that PS PDF contains. They do seem to be large and Gimp might be smaller, as long as you remember the Gimp PDF is now a single bitmap image.

EDIT Just seen Windows XP and Gimp 2.8 - All as above except text is automatically flattened and there is no mult-page option anyway.