What's more important is what the pixel dimensions of your images are. "300 DPI" is a useless parameter if you want a 6" x 4" print from a 600 x 400 pixel image.
On the other hand, if you have an 1800 x 1200 pixel image you can produce a "high resolution" 6" x 4" print, even if the image metadata says "72 DPI" in the little box. You just change "72" to "300" in the print dialogue and bingo, you get your 6" x 4" print.
In other words, if the pixel dimensions of the images you've supplied are adequate to produce the size you want at 300 DPI (even if the images are "at" 72 DPI), they can change it themselves at no detriment to the final result.
I did say ppi, not dpi. I kept most of the files the same size and changed the resolution to 300ppi. Some were made smaller in size and changed to 300ppi. The images that were most affected were new photos received from my client as .jpg.
The same argument applies. It's the pixel dimensions that matter, not the number in the resolution box, which can be changed after the fact depending on the size of the printed output you need.
Whilst not strictly correct (and mea culpa), DPI and PPI are often used interchangeably. (DPI really refers to ink dots per inch).
I guess I still don't get it, as I didn't want to change the dimensions of the image, just the resolution. If anything, I made the dimensions smaller, and the result looked better to me than the original.
Could my client's text affect the page if it is less than 600 dpi?
Ppi only has meaning in the context of a given physical print size. Pixels per inch. Do the math.
If they say it's not 300 ppi, they either mean not 300 ppi at the required size - or they don't know what they're doing (it happens). If it's a book, the photos end up placed in an InDesign file. There, you size the image on the page as intended, and then you read out the ppi at that size. That number should be 300 or above.
I guess you could ask the print shop which files they are, check the print spec of each file under Image > Image Size, correct if need be and resend.
If you do not want to alter the pixel dimensions, uncheck "Resample" and input "300 ppi", and your "document Size" will show you what the Image will print out in inches will be at 300. That's called "re-scaling". If that's 3 x 2 and you need 6 x 4 @300, you will have to check resample and increase the number of pixels (not good).
I don't know what method of transport you use, but some email clients actually take out the print metadata.
Thanks to all who answered. I guess Imagenomad's response most correctly mirrors what the situation was. My client was resizing the images inside Word, and often was increasing the size to fill space on his page. Regarding calculations, I chose to use Perfect Resize(R) rather than do calculations myself. It takes a long time, but it does the calculations for you. In the end, my client received his proof as it was and approved of all of the images, so he is happy.
Great. Please mark Imagenomad's response as corrrect and mark your question as answered.