Much depends on the particular word processor used because of how word processors export files to PDF.
For Microsoft Office, “Select All” will not include inserted images, shapes, and numbered lists or bullets. Each object will have its own font and those would have to be updated to accepted fonts and sizes. Be mindful of images inserted into shapes, as each image and shape can have its own font type.
In Microsoft Office Word, double-click an image to ensure it was not inserted as an object (e.g., PowerPoint slide). If another application opens, this means a non-graphical image was inserted and the object contains invalid metadata that will make the PDF upload fail. Re-insert an image by pasting it as an image.