The only application that can open a Pages document is Pages (Mac, iOS, iCloud), or Preview. You can however drag/drop vector .psd and .ai files into Pages. There are factors within the Pages document, and controllable by you, that govern the sharpness of the resultant PDF.
To JPEG from Pages
File menu : Print : PDF : Save as PDF. This is the sharpest that Pages can make the PDF provided that the Pages content has no transparency in it. Once you have the PDF, and still decide that you want it in JPEG format, you can open it in Preview, switch on thumbnails, select a single thumbnail, and File menu : Export… . If you press the option key, the export image format selection offers more image types.
Factors that decrease sharpness in a exported PDF
Pages is a word processor. Although you can create and incorporate artwork within it, you must take pains to not introduce transparency, or it will render that content at 72 dpi, and diminish the perceived sharpness in the exported PDF.
- Text boxes have inherent transparency.
- Any reduction in opacity from 100% with any document object.
- Any attempt to layer images with varying opacity.
- You are not in control of your color model in Pages v6. It will change CMYK to RGB at every opportunity.
- In the Image tab, borders, drop shadows, reflection introduce transparency and resolution decline.
- Low resolution images stay low resolution. Screen shots are 72 dpi.
- Keep your artwork with text purely vector from Photoshop, illustrator, or other purpose built vector pro tools.
Any reason you are not producing your flyers and tri-folds in Illustrator and keeping it strictly CMYK and vector? Pages is not Quark or InDesign, and will resist your efforts to make professional appearance results.