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When you export photos from Lightroom, there is a check box in the Metadata section of the Export dialog labeled "Minimize Embedded Metadata," which is a source of some confusion for many. Let's take a look at what that little check box is all about.
The intention behind that option is to provide you with a means to strip out all the metadata in the file with the exception of your copyright information during export, and with JPG files it does exactly that. The confusion arises when people fail to realize that "all metadata" includes keywords and the resolution tag.
As a result, an almost daily question on Lightroom related forums across the net goes something like, "How come my exported files always default to 72 PPI no matter what value I enter?" The cause always turns out to be the fact that they checked the Minimize box. It is important to note that the pixel dimensions of the exported file are exactly what they should be, all that is missing is the metadata tag that tells the printer what the PPI (pixels per inch) is set to in that file.
So, if you want to keep that resolution tag in your exported files you can either uncheck the Minimize Embedded Metadata box, or check the box and add the resolution value back into the file after export in a program like Photoshop.
If you go the Photoshop route, open the photo and go to Image > Image Size, uncheck the Resample Image box and enter the desired resolution value in the Resolution field. Click OK and Save the file.