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As mentioned above, creating an accessible PDF usually starts with the source document which created it. We'll outline a few common practices and the way to edit a compiled PDF if all else fails here. One side-effect of the plethora of ways you can create a PDF is that some PDF creation suites might not have the tools to add some of these recommendations natively, and in those cases you'd have to similarly remediate them in post compilation software like Acrobat.
Acrobat
Adobe Acrobat Pro is likely the most complete suite for checking and amending PDFs.
Automatic Testing
Acrobat Pro will give you an automated overview of the document’s accessibility health, like lighthouse or AXE browser tools. The following guide will produce a report of actionable fixes.
Go to tools
Add the Accessibility Tools Shortcut to your tool bar
Click the Accessibility Tab on the right
Select “Accessibility Check”
Now, on the left of the document, you should see a tree with what the parser thinks the document structure is and any issues it may have caught
Remediation
In Acrobat Pro, you can remediate most issues using the tools available within the Accessibility suite.
On the left is the bookmarks, or what the PDF viewer interprets the document structure to be.
And on the right hand side, you can add alternative text to elements that don’t have them.
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Word
Microsoft Word
InDesign
Adobe InDesign
Acrobat
Adobe Acrobat Pro