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Create a bottom layer with a white rectangle, then export to PDF.
(If the poster was designed in Illustrator without a white rectangle as the bottom layer it will automatically save as a transparent background. This will crop your poster to the closest object getting rid of any margins you may have. Placing a white rectangle as the bottom layer preserves the margins and dimensions of your poster.)
Export your file using XeLaTeX, not the default PDF LaTeX (Menu --> Settings --> Compiler). If using Overleaf, make sure to clear the cache as well.
Open the PDF and check that all the information has transferred properly before submitting your print order!