To add to @JohannaH's great list of resources, for anyone new to PDF accessibility and would like to know more:
AccessAbility 2: A Practical Handbook on Accessible Graphic Design | RGD
A good and brief overview of how typography, colour and language affect accessibility (digitally, and in print)
It has some short guides for making accessible PDFs from InDesign or Word (+Acrobat)
It also has a really useful section on how to convey the benefits of prioritising accessibility to bosses, stakeholders and co-workers
As a side-note, the PDF document itself (now) also functions as a good example of tag structure and use of alt text
WebAIM: Alternative Text
For me, alternative text (or alt-text) is frequently the part that stumps me the most
WebAIM's guide gives a good outline of the "What", "When", "Where" and "How" of writing alt-text, with examples
A good resource for troubleshooting!
Last of all, my biggest (subjective) tip – sometimes the most accessible PDF may be no PDF at all... by which I mean:
Explore what the best format(s) for the digital output of your project is going to be, before defaulting to PDF:
A well-structured HTML document is the gold standard for accessibility, and offers the most flexibility for users to tailor the content to their specific needs
A "reflowable" EPUB3 is basically a portable HTML document
For your content, there may be fewer steps involved in making a well-structured and accessible HTML or EPUB3 document
Tools for creating accessible HTML and EPUB3 documents are far more widely available (and free)
As a user-friendly way for writers to generate structured HTML documents from plain text, consider incorporating Markdown – as used by the online tool AROW, for example
Not everyone can afford a subscription to Creative Cloud (or even Acrobat Pro on its own)!
Only share information as a PDF if you absolutely have to
...and provide the important information in an alternative format, such as – you guessed it – HTML and/or EPUB3
Not to say PDFs aren't useful (they're not going anywhere) – but asking this question at the beginning could save you a lot of unnecessary headaches later.
Also – as @JohannaH highlighted, and many weary PDF remediators will tell you – Acrobat Pro and InDesign / QuarkXpress / MS Word don't represent the best, simplest, or most intuitive processes for creating accessible documents.
They are just the least-worst tools available, and these are just useful guides for making the most of them
Keep pushing for better tools!