There is no simple answer to your question, Lina. Other than, maybe, “it depends”.
PDF is just a “container” that you can use to store a lot of different raster (JPG, TIF, PNG) and vector (SVG, PS) objects plus clear printing instructions. If you wrap a PDF around a JPG image, the PDF will not be much larger than the JPG. So the 3MB JPG file might become a 3.05MB PDF but not much more.
When you talk about 20MB PDF files, the embedded image is most likely a TIF encoded image. The reason that TIF is often much larger than JPG is that it is a loss-less compression format. When you read or print a TIF file, you get exactly out what you stored in it.
JPG on the other side is a lossy compression format. In order to make smaller files, it reduces significant amount of image information that you do not get back when you read/print the file. This is done in a way that for most images, most people will not see any difference. But some images suffer from very ugly compression artifacts. That is specifically true for color gradients (large areas of very similar colors) such as skies and sunset/sunrises. And the problems gets worse if you read-change-save JPGs several times. Just like video tape copies in the past.
And finally, you can select a “file size” or “quality” when you save JPG files. Highly compressed JPG images will look great on screens (computers, phones, pads) but might print very ugly. On the other side, even a 100% quality JPG removes more than half of the colors from most images (that's how JPG compression works).
So the decision is probably first JPG vs TIF, then resolution (ppi/dpi), then whether to embed the image in a PDF.
We always encourage our fotoflōt customers to provide their art to us as TIF files or TIF-in-PDF files. To reduce file sizes, TIF files can be compressed with the LZW or ZIP methods, which are both loss-less. We have seen too many stunning images with very ugly JPG compression artifacts in prominent areas. And our stunnables products here in Etsy have been processed and are stored in 16-bit LZW compressed TIF for superior quality.
As to the resolution, interestingly you'll need more resolution for smaller images than larger ones. That is because you will be much closer the small images than large ones. 300 ppi is probably more than enough for your art. Please observe that I said ppi and not dpi. You provide “pixels” to your customers so that the correct measure would be “pixels-per-inch”. When your customers print your art, the printer may use one of many techniques and will create dots which are measured in “dots-per-inch”. An ink-jet printer, for example, make thousands of little dots just to create one pixel in your file and might have a resolution of 9,600dpi. But you (or your customers) do not have to know about that since the printer takes care of that.
With all that in mind, I think 300ppi is probably fine, 100% JPG quality file will probably work for most of your images, and PDF embedding might help some of your customers to get better print results. I have not worked with Etsy digital downloads yet, can you offer both (PDF and JPG/TIF)?
And one final thought. It is imperative that the files your provide to your customers contain an ICC color profile so that color in the prints your customers make/get are anywhere close to those on their screens. I suggest your keep the files in the RGB color space with an sRGB color profile.
Hope this helped!
Jürgen
PS. Have you considered image sizes that are more common here in the US? Such as 8in x 10in, 8.5in x 11in (letter), 11in x 14in?
