Dear Dave! Thank you very much indeed for your explanations! The file was originally exported from AI though SVG format. As for vector parts of the project - indeed, when I start to work with them directly in Affinity and then export to PDF, everything is OK! So, now I understand how to cope with this.
However, my question also regards the rasterised image of tea itself. Despite all my attempts to change the export resolution of this slice in "export options" I still get very low resolution in the final PDF. What could be a problem here?
Many thanks again,