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Open your document and click 'Save As...', then select PDF.
You should see a box that says Options... below the main save menu.
Click on this and then check that the 'Document structure tags for accessibility' box is checked.
If you have selected 'Minimise size' to create a PDF with a smaller file size, make sure to double-check that the 'Document structure' box is ticked.
The reason is that minimising the size of a document can sometimes uncheck the 'Document structure' tick box.