If you're scanning or trying to save a PDF and you get a "PDF/A" message, first realize that this isn't an error, but rather a warning. You're getting it because you selected the "PDF/A-1b" option when you scanned or saved the PDF.
PDF/A stands for "Archival-quality PDF". It's a special type of PDF which is supposed to be readable for at least 30 years. To make that possible, the PDF/A format dictates that only a certain subset of the regular PDF features is allowed.
This message is a warning that if you make any changes to the PDF, it could jeopardize its status as a PDF/A.
If you find the message annoying, you can select the option to not show it any more.