Is this happening with all PDF documents, or just with one or a few coming from the same source?
Do you see the same effect when exporting to other image formats?
What application is showing the "stretched" image?
I suspect that you are dealing with a case where the pixels in the source data are not square (a different resolution in x and y directions), and the PDF generating application stretches the image in the direction where the pixels are shorter to make the whole image appear correct again. This information can be encoded in the TIFF image, but without running some tests, I don't know if Adobe Acrobat actually does that when exporting TIFF images.
This is happening to some of the PDF documents. I have about 30 InDesign documents with the same source of collection of images and when exporting to PDF they all look perfect. When I export to Tiff, Jpeg or PNG some of these get stretched.
It doesn't matter if I open the files in Preview or Photoshop. They both look the same.
Liked your theory but when I looked at the source image the pixels in it are square. And if it would have been in the source image then the text should not have been stretched, correct?
P.S. And thanks for thinking with me.
Let me first say that I might have misunderstood the question: You are talking about exporting PDF files as TIFF images, I understood exporting all images in a PDF. That changes things a bit. The following is still about my interpretation (I wrote this before I realized that I might be looking at this the wrong way):
The text is part of the image (otherwise it would not be exported as part of the image), so it would stretch too.
However, I just ran an experiment with an image with a pixel aspect ratio of 1:1.33 (100dpi and 133dpi), placed that into an ID document, adjusted the aspect ratio of the image so that it looked square again (it started out as a square image), exported to PDF, and then exported all images from that PDF, and I ended up with a square image with square pixels (233dpi in both directions).
And now, that I may actually understand what you are doing, let's take a look at this from a different angle: Which version of Acrobat are you using, and on what operating system (your mention of Preview suggests that it's on a Mac, so which Mac OS version)?
When you bring up Acrobat's Preferences and then go to the "Export from PDF" category, select your output format and then change the configuration settings one by one, do you see a difference in the image output?