The journal does not want pdf nor 300dpi. They want 1000 dpi. How do I get there from mac powerpoint?
Journals ask for esoteric and often silly image requirements all the time*. Unfortunately you're starting with an image in PowerPoint. It isn't a graphics program though, it's a slide deck program and your options are limited. This time, you will need to export as a vector (PDF), open in Preview, and then export as the DPI desired by the journal.
Start here for help: Changing the target DPI when saving slides as images in PowerPoint 2011
Note that if your image is just that - an image, NOT a chart or graph or text, but literally a photo - exporting at higher DPI will just give you a blurrily-enlarged version of your current image, NOT a crisp, clear, high-resolution file. In that case, you'd need to re-create your image at a higher resolution starting from the original source files, which (if they are not at or above 1000 dpi) might literally be not possible.
...and next time, use a proper graphics program to create your images. If cost is an issue, open source ones exist, such as Inkscape or GIMP, that will get you closer to what a journal wants when it comes time to publish. The MS Office suite is ubiquitous but that doesn't mean it's the proper tool for the job.
(*The journal requirement is silly, really, because PDF is a vector format - which means it's resolution-independent, and looks good at any size. Most journals prefer vector images. Some refuse them, for no good reason. The ONLY time a vector isn't superior is where the original artwork is something other than lines and curves, such as photographs/photomicrographs. For graphs and charts, vector is much superior.)