Preview Changes File to 72 dpi upon opening

DPI is meaningless until a print size is selected. Determine what is the largest print size that will be required of the image and create a image with the pixel dimensions that meet that requirement, i.e. if it's an 8" x 10" print give them a 2400 pixel x 3000 pixel image.

So no matter what is shown as DPI it's the number of pixels and print size that is important.

This website explains it in more detail: The Myth of DPI

OT

Is this in Illustrator file you created as vector art, or does it contain an embedded raster image?

If the former, vector art has no resolution at all. It's a construct of mathematical points, curves and fills. That's why such files and fonts (which are created the same way) are considered resolution independent. When you open your Illustrator file in Preview, it is assigned a generic "resolution" of 72, but is entirely meaningless unless you save the file out of Preview in a raster format such as TIFF or JPEG.

If the latter, why would you embed a raster image in a vector art layout? Unless you have both types of components in the art (in which case an Illustrator document is the only way to go), it should have been created in Photoshop or other raster image editor and saved in a raster format. If it is a blend of vector and raster art, then easiest way to send out the final Illustrator file out is as a PDF, so the raster and vector art stay in their respective formats, but are then blended all into one file type.

The page Old Toad linked to has numerous errors regarding DPI. Right off the top, it's a mistake to declare the image of the house as having no difference. The web doesn't give a hoot about any type of image resolution. The number of pixels in the height and width is all it cares about (as does video production).

So to say there's no difference is wrong. Of course there's a difference. Not on the web page since it's only displaying the images but their pixel count, but if you print both of them independently of the web page, you wouldn't be able to miss it. The 1000 DPI file would print as a spec, and the 72 DPI file larger than what it looks like on your monitor. The resolution tells the printer, or other output device how many pixels to use in a linear inch (DPI) or centimeter (DPC).

There are other errors on the page I don't have time to get into.

The page Old Toad linked to has numerous errors regarding DPI. Right off the top, it's a mistake to declare the image of the house as having no difference. The web doesn't give a hoot about any type of image resolution. The number of pixels in the height and width is all it cares about (as does video production).

Isn't that what the author is trying to get across? Those two pictures are described at the bottom:

Test 1: this was the first photos on this page, both 300 pixels wide, one with a setting of 1000 dpi the other with a setting of 10 dpi. The test answer is that they are identical in digital resolution.

They are the same pixel dimension, 300 pixels, but are at different dpi when viewed by an application like Photoshop or Preview t. Same pixel dimensions but different dpis. They are only different when printed to different print sizes.

Isn't that what the author is trying to get across? Those two pictures are described at the bottom:

Probably, and I grant there isn't an easy way to say what he meant. Most folks in digital imaging only use the word "resolution" to describe the output value. i.e., 300 dpi, since output is the only place resolution even means anything. If you want to know the "pixel dimensions", either that phrase is used, or something like "image size". The latter itself is vague since you may get an answer of pixel height/width, or the person may tell you how many inches Photoshop says the height/width is. Though the second is also meaningless since changing the resolution (without resampling) changes the dimensional size (inches).

Since you have a blended file (vector and raster art), and if the service won't accept a PDF for output, then you need to get it into a single file type. Usually, that's a JPEG.

What I would do then is drag and drop the Illustrator file onto Photoshop's icon. You will be presented with this:

Bounding Box will confine the converted image right to the edges of any live art, disregarding the page size you set in Illustrator. If you want it to be converted to the page size, chose Trim Box.

Make the resolution and color mode whatever the service requires. Make sure Anti-aliased is on, or any vector shapes that are not perfectly horizontal or vertical will be converted to raster with jaggy edges. Any embedded raster art that is not placed at 100% of size, or is not the same resolution as the converting res you set will be scaled without smoothing if that check box is off, and will not look good. Click OK and PS will rasterize the entire file into one layer. Save the image out of PS with a new name as a JPEG, or whatever format the service wants. If JPEG, save with the highest quality setting (12).

Ah! Easy then. 🙂