does Hydrochloric Acid react with PET? From someone coming from electronic engineering not skilled in chemistry,

There are transparent electric conductors in form of ITO ( Indium tin oxide ) coated glass or transparent PET films ( Polyethylene terephthalate ). By etching the ITO coating we can create transparent circuit boards and with the case of ITO coated PET also flexible transparent circuit boards.

There seems to be very little info on what can be used for etching ITO PET sheets.
For ITO coated glass, people use HCL with no issue, but will HCL also work to etch ITO but leave the transparent PET film intact, or
do PET and HCL react?
You can etch ITO on PTE by mean of conc. HCl solution without problems. The treatment duration depends on the ITO thickness but is limited to minutes.

Just make few samples (residuals from cutting etc will suffice to make this test) to find out how long it takes to fully remove ITO.

No reference need it as I do etching of ITO coated PTE electrodes since years.

For sake of completeness let me add that I never investigated PTE itself prior and after the etching so I do not know if there are changes which, for instance, may interfere with long (very long) stability etc.

In my case it suffices that PTE maintains its transparency and still behave as a robust but flexible substrate. It seems it could be your case, too.

Just to be pedantic. Obviously after etching the foil must be washed thoroughly.