Some images that look good at 100% on screen have very fuzzy edges, fuzzy type, or pixilation on paper. Screen displays work differently from printers. A monitor can display a small image at an adequate screen display size, while a printer has to deal with the fact that the amount of information is much lower for an image that is too small for a full size print.
A good way to test an image before working on a final document is to put a copy of your image onto 2 separate flyers in the program that you will use for your document. Make one of them small and one of them large (so it almost covers the page). When you zoom in to 100% or so on both of them (using the image display software on the computer) you may notice that while the small one looks very sharp, the large one is showing square pixels or a very fuzzy transition. This means that the image resolution is not adequate for that size.
The resolution is often quoted as a resolution measured in dots per inch or DPI, though in fact it should be pixels per inch (or PPI). When your image is placed in a layout and resized to be bigger than original size, the pixels are spaced further apart on the print and therefore resolution falls. The original image may have had enough pixels for a business card, but not for a full size poster. It is not enough to look at the image dimensions alone; check image resolution at layout size.
Another confusing issue is that photographs are raster images made up of pixels. If they are scaled up, pixelation or loss of detail becomes apparent, while vector graphics (logos and simple line drawings) can be scaled up without pixelation. So, it is much better to get the originals for logos etc in vector graphic form, instead of a raster graphic. A logo which is low resolution raster in your layout program cannot be made to appear sharper by saving to PDF; it was not a vector graphic when imported, only when exported to a different format, and that will not give a better original graphic for scaling.
PDF export settings in the software will also affect your image quality. Often a default export setting for PDF files will either compress or reduce the resolution of a photo in order to give a smaller file. That is probably OK for an email, but not for a print ready file. Check image compression/dowmsampling settings in your export options before PDF export, then open the resulting PDF in the software you view files in and examine the zoomed in detail at a high zoom level. You may well find some of the image quality has dropped and you would have seen detail lost in the document.
You don’t have to make every image a massive file size. Base your resolution on how big your image needs to be for the final print, keep logos etc in vector form as much as possible and don’t expand small raster images too far. Remember, the test of your image improvement is not that it looks sharper when you zoom in to 100% on the computer screen, but that it stays clear at the print size of the layout.