Within a PDF or even stylized text on a Web page, an important accessibility question is whether a piece of verbage is really text or is an image of text. For accessibility, this is important for determining whether you need to worry about the ALT tag or not.
A simple text for differentiating text versus image is to move your cursor to the text (mouse or arrow keys), then selecting it (mouse or SHIFT+arrow keys). If you are able to select/highlight each character one by one, then it is text.
However, if your cursor highlights the entire text piece all at once, then you probably have an image. For a PDF based on a scanned document, that means you won’t be able to select ANY text on a page, which can be inconvenient for both sighted and unsighted users.
WCAG 2.0 Note
One item from the WCAG 2.0 accessibility guidelines is that if you have a choice between using stylized text or an image, choose text. Not only does it make accessification for screen readers, but it reduces file size and makes the page more accessible for low vision users.
WCAG Guideline 1.4.5 – If the technologies being used can achieve the visual presentation, text is used to convey information rather than images of text except for the following