What about the document resolution?

In general, digital printing does not need extreme resolutions. For photographs, make sure the resolution of your file is over 150 DPI at the maximum print size. For other images, especially with text or line art, use a resolution that is higher: say, about 220 DPI at least.

In PDF documents, the same rules apply. Take note, that the dimensions of the document define the maximum print size, so the DPI of any embedded images must be calculated with respect to the height and width of the source PDF. Please use text for text (so do not convert your text to images - that has nearly always a bad result).

Even if you are not the most experienced publisher, you can use a simple life-hack to identify low resolution. You can use a program like Adobe Acrobat to open the PDF on a computer screen. This is usually displayed at 72 DPI. So, if you zoom in by 200%, it should show the result at approximately 150 DPI. If you see pixelated images, your images are not suitable for print.