Tab order in pdf doesn't work (no fields or forms, just text blocks)

Thanks for the detailed screen shot, @YvonS. It explains what you're trying to create.

Examining this from an accessibility viewpoint, you have a table of information that should be arranged in a spreadsheet matrix of columns and rows — there's the WAT column, and the HOE column that is further broken down into 2 sub-columns.

It really shouldn't be individual text blocks that you then have to wrangle into a correct Tag reading order and Architectural reading order.

A table structure is required because the user must be able to read down the first column and choose either Documents, E-mails, SMS, or WhatsApp, and then read across the row to find the screenshots or video instructions. This is exactly the correct functionality that a table structure gives end users. Plus, your reading orders are set perfectly!

I quickly created this sample that mimics your design (both the InDesign and PDF files are attached).

The key is that I created a table that sets the correct accessible structure for the content. And then I created Paragraph Styles to format the heck out of it. I heavily used the new Paragraph Shading and Borders settings in paragraph styles to create your design of the blue "hotdog" boxes.

By using Paragraph Styles to do the visual formatting, most of the text boxes are eliminated and your reading orders are left in tact. I did nothing to the PDF, other than to do the usual minor corrections to table span settings, which took all of 1 minute.

... Do you see a way to make this simpler? 

By @YvonS

Yes.

1. Minimize the number of frames in the design. Use background shading on paragraph styles where you can to create faux boxes that look like frames but aren't. Example, eliminating the 2 white "column" boxes and use cell formatting instead to make the 2 sides clearly visible. The blue hotdogs could also be replaced by simple cell formatting, too.
2. Simplify the design. The design instrustry has been moving to a cleaner "flat" design aesthetic that uses flat color blocks, fewer rules/borders, and simplier designs. As a creative director of a small cadre of designers, I like this movement!

As Mies van der Rohe said, "Less is more."

And as Bevi Chagnon says, "Not only is 'less' more, 'less' is more accessible." <grin> (I have a degree in architecture and studied the philosophy of Mies and the Bauhaus movemement. Their theories have greatly affected my graphic design career ever since.)