You have an Adobe Illustrator document you've spent a bit of time on, drawn the artwork, laid out the artboards and clicked Pixel Preview mode before exporting it to a raster document, but the PPI (Pixels Per Inch) is set wrong so the resolution looks completely wrong! How do you fix this? Where is the PPI properties screen?
I hear the vector fanatics yelling at me already: PPI doesn't matter in illustrator! It's a vector tool!
But there are times when it does matter for certain applications. Perhaps you're working on some pixel-perfect icons and need them to work on a set of platforms with different PPI screens. Or perhaps you have some vector plans at a certain scaled unit size, and you need a raster at the right PPI to show the details you want. In my case, it was the latter to use as a background guide to make a 3D model.
Slow Solution: New Document
One option is to create a new document and set the right DPI (New Document > Advanced > Raster Effect), but this means setting up your artboards again, copying the artwork across, and scaling it for the new size. This approach is also limited to three options for PPI (72 ppi, 150 ppi, and 300 ppi). If you want a different resolution or want to try a few resolutions, you're out of luck.
Dirty Solution: Export Settings
Another option is just to ignore Pixel Preview and change it in the export settings when exporting to the raster document. But if you want pixel-perfect results, this doesn't really work well without a lot of exporting and checking results and tweaking.
Better Solution: Rasterize Effect
The best method I've found is slightly hidden. Whilst unfortunately it doesn't change the "pixel preview" view, it still gives a preview of the pixels.
Select all of the artwork, and then in the Effects menu select Rasterize. Here we can choose one of the default PPI options or enter a custom value. After hitting OK and waiting for a short progress bar, you'll see all your artwork rasterized at the PPI of your choice.
To alter this, navigate to the Appearance panel and you should find the Rasterize effect. From here, you can switch back to the vector view by clicking the eye icon (to hide it) or the bin icon (to delete it) or click on the name to choose a different PPI.
You've seen a few different solutions for changing the PPI of an existing document in Adobe Illustrator. Hopefully these solutions will help anyone with similar problems! But if you know of any other solutions, please let us know in the comments below.