Jagged lines is a hallmark of low resolution, not file type, or a lower quality screen preview than you think you are using. Check the Display performace settings again to be sure they really are correct, and if they're set to allow object level settings, you'll need to verify that each object is also set correctly.
Beond that, saving as .jpeg is always destructive. For the web, that's fine, but for print you should never use anything, in my opinion, but the highest quality settings for saving jpegs, and I advise again using jpeg at all. If you image is all raster data (pixels) save as .psd or .tif. If it has live type or other vector content, save as Photoshop PDF. In Illustrator save as .ai or PDF with Illustrator editing capability. If you change the extension of the Photoshop PDFs to PDP they will open in Photoshop instead of Acrobat when you use Edit Original.
As I said, the jaggies are a hallmark of low resolution. We often have people tell us that the file is 300 ppi, but it turns out that's at some very small dimension and the imaga has been scaled up in ID. Both the Links panel and Info panel can show you the resolution information for your images. There will be two numbers, actual and effective. Effective resolution is the only number that matters. For printing on a desktop printer I like to see a minimum of 150 ppi, but you might get away with as low as 90 for a non-critical image. For press 300 ppi would be the rule of thumb, and dropping below 200 is asking for trouble.
How the file looks in ID is not nearly as informative as how a PDF exported from the file looks, or prints. For a high-res image in ID you are still looking at a lower resolution screen preview, not the actual image as it will be output. Export settings that downsample too far or apply too much compression to reduce file size can also turn a good document into a clunker.
Thanks. When making the images in PS, ID and AI, I've set the canvas/page/artboard to the size I think I will use the final image, at 300 dpi. Is that correct?
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Illustrator files (vector) have no resolution, so they can be scaled in ID with impunity (to the point where strokes become too thin to reproduce), so you can work at any convenient size. In Photoshop your method is sound, but not essential, as long as you understand that scaling in ID changes the effective resolution in reverse proportion to the scaling percentage. An image with too much resolution is usually preferable to one with not enough, so better to err on the side of too large, without having to upsample.
Thanks. One last question: I often make ID documents that will be placed into other ID documents. How should I save them?
The same way you’d save any other file.
The only reasons I can think of NOT to save them as .indd (which means only one step if you need to edit) is if there is a chance the file will be back-ported to an earlier version via .idml or if the page size will be very much scaled down -- I've seen a case where text in the placed file reflowed do to loss of kerning information at the versy small size the type became whenthe page was scaled inside a new document. Exporting PDF has proved flawless for me on both counts, but it means you have to re-exprt the PDF if you change the .indd that it was made from.