Are you working with a colour profile? Do you have any effects applied to the pages in question?

Ah! I think I've got this one.

Possible cause A. Is your text that looks significantly darker closer to an image than the other text? (That would explain it being slightly "heavier.")

Possible cause B. Are you using more than one "black" swatch, or maybe the [Registration] swatch in InDesign? (That would explain it being slightly "darker.")

Cause A would be because text that is under transparent portions of images is rasterized. Cause B would be because "rich black" looks darker in PDFs than "pure black" and sometimes prints that way, depending on the printer etc.

Go to InDesign preferences > "Appearance of Black" and make all blacks appear accurately. Can you see the problem in InDesign?

When I'm having an InDesign problem I can't solve, I try a process of elimination. Make a backup copy of your file; then take a couple of pages where you're having the problem and temporarily delete every object (photos, rules, background tints, etc) except for the text boxes. So you just have the text boxes and nothing else. Then see if you're still having the problem. If this fixes the problem, you know some interaction with another element is causing the problem.

Thank you for your comments guys. I tried changing the appearance of black. That didn't fix the problem. I tried moving around some pictures (I have pictures or shapes on top of some pages) and it did change the color to darker, when ever I placed a picture on top of the page. So, Joshua, I think it's Case A. Any suggestions how do i solve it so everything looks the "same black" in pdf? Big thanks!

p.s. When I delete the image from the page, I can visually see that there is a change in the text (in InDesign when zoomed out, I delete the image, and the color of the text lines turns from light gray to dark gray). So this is definitely connected with the images.

I was playing around and found, that adding transparent text or a transparent box somewhere in the page, makes the text turn out darker in the PDF.

Ok, so sorry about the spaming. I came up with a solution (probably not the best way to do it, but it works). I just added a dot (with transparency set to 0%) at the Master pages. Now every page has a transparent object, so the text in every page will seem "darker". If you have a different solution, please do tell me.

Thank you all.

That's clever. Or, you can just make sure that all your text is on a layer above all your images, which I believe should also work. (Although then you might have to worry about trapping …)

I'd go with Joshua's suggestion of putting the type on a separate layer. In fact I know some designers who do that routinely as a precaution against various problems.

It's pretty easy to do. Open the Layers palette and create a new layer and name it "Type". Then with the palette still open, click on your text boxes to select them. (If your text boxes appear on your master page, you can do it there rather than on each individual page.) In the Layers palette a little box will appear on the right side indicating which layer those objects are on. Drag that little box to the Type layer.