For better or worse, the term flattening has any number of meanings in terms of PDF files and the content thereof.
(1) Flattening can refer to combining multiple layers defined in a PDF file into a single layer.
(2) Flattening can refer to transparency flattening which refers to eliminating any non-opaque objects by converting same into comparable opaque objects. Within Adobe's Acrobat products, flattening typically refers to transparency flattening and nothing else.
(3) Flattening has also been used to the process of removing “live text” within a PDF file and converting same to outlines or possibly raster images in the misguided belief that this will prevent someone from modifying the PDF file's text content.
My belief is that the “building departments” you are referring to really mean (3). However, anyone, including whatever “Adobe Chat Line” you are referring to who tells you that “printing” the PDF file (presumably to another PDF file, a process known as “refrying a PDF”) is going to somehow “flatten” all text is dead wrong. All that will do is flatten transparency that may convert some text to outlines or rasters if such text is involved in transparency.
If the “building departments” in question will accept text converted to outlines, there is a simple fixup in Acrobat Pro DC's Preflight, Convert fonts to outlines, that will accomplish that in one step. Very easy. It diminishes text viewing and printing quality but not nearly as much as converting text to raster images which really can dramatically clobber viewing and print quality and increase PDF file size.
Ironically, none of these procedures, including those proposed by others in this thread can prevent someone from opening a non-text PDF file in Acrobat and using the edit function to delete or cover the “flattened” text objects, adding new/changed text over the original contents, and then reflattening the document.
The only protection that can begin to work is to password protect the PDF file against changes and then add a password to simply open the PDF file. Unfortunately, the “building departments” don't quite understand this at all.
On behalf of Adobe, I apologize for any misinformation you may have received from any Adobe source.
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