To upload header and footer images for your document PDFs, go to Company Settings > Basic Settings > APP SETTINGS > PDF SETTINGS > turn on Setup own header & footer images > Choose images from your computers > Save

WARNING: Once this setting is enabled, company logo will not appear in the PDF

Please note that the setting requires users to upload both header and footer image and it is suggested to follow the image size of 1500px x 210px; max 2Mb