I've received a batch of PDFs from a designer and am charged with making them accessible. I've added tags to about 25 PDFs without a problem, but encountered this issue twice. Here are the footer images in the PDF prior to clicking Add Tags to Document:
And here's what it looks like after clicking Add Tags to Document:
It appears the images are comprised of many individual elements, some of which are removed in the tagging process.
Does anyone have any suggestions for preventing this kind of image corruption? Perhaps it's an issue that has to be addressed prior to PDF creation. Any help would be appreciated.
Yes the images are made up of many pieces -- either font elements or the fonts have more likely been converted to line objects or "curves". In the graphics world these are considered vector objects (EPS is one common format that is used to contain vector art).
I've seen this kind of problem before. I don't know why it happens, but it does. For this reason, in Tagged PDFs it is always advisable to convert vector objects like this to a bitmap format -- JPG or PNG work well. It may be best to do it in the original file, particularly if the image is in a footer or header.
But you can also do it within the Acrobat PDF. There are several ways to approach the problem... I like to extract the page with the graphic, crop the extracted page so only the graphic is showing, then Save As PNG. Then go back to your original file, delete the problem object and Insert the PNG. You can adjust the resolution when you Save As, to make sure the quality is comparable. Vector objects are usually sharper than bitmaps, and they retain their sharpness at even the highest magnifications. For your PNG or JPG choose a resolution of 300 dpi and the PDF should be suitable for onscreen viewing and laser printing.
Thanks raeben3, this is good information.