In QGIS 3.0 the print layout window shows the following:
The result after pdf exporting is this:
How can I prevent the downsizing?
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If you want to stick with annotation, then you can double click the annotation, select the text, and increase the font size until you are satisfied with result.
Change the Font size of the annotation:
In QGIS 3.4 it works as explained in QGIS 2.18.
Output PDF (small font):
After increasing the font size:
Output PDF (big font):