1. You must set the Sheet Layer dpi at 300 or higher if you want to get a true 300dpi image from it.
2. You can create a custom sheet size in Page Setup to contol the final image size. Below I've created a 22x34 Custom Page Size to set the final imported image size to 34x22.
7.
Here is a copy of "Open JPG 300dpi in Preview" workflow I have installed in my PDF Services.
You can import it into your PDF services by selecting "Edit Menu" in "PDF" Print window menu shown in Image 2. above.
And selecting the unzipped file below.
Open JPG 300dpi in PreView.zip