How do I open .tax2017 file when using the 2018 web app?

See this TurboTax support FAQ for transferring 2017 tax data to the 2018 online tax return - https://ttlc.intuit.com/questions/1900883-how-do-i-transfer-last-year-s-return-into-turbotax-online

This is not an answer.  I want to upload my 2017 CD/download tax file to the online web app.
I cannot recommend this answer.

The answer is correct.  All you needed is provided in the section - "If you used the TurboTax CD/Download software last year"

You can transfer last year's tax data file (with a filename extension .tax2017) if you're preparing your 2018 return online in TurboTax Live Basic, TurboTax Deluxe, TurboTax Live Deluxe, TurboTax Premier, TurboTax Live Premier, TurboTax Self-Employed, or TurboTax Live Self-Employed. (Free Edition, with or without PLUS, doesn't support transferring last year's .tax file.)

You won't be able to transfer a PDF, it must be a tax data file with a filename extension .tax2017.
Remove any password protection from the data file before transferring. Instructions
Note: If you already started your 2018 return in TurboTax Online, transferring will clear any info you have already entered. Please keep in mind, a .tax file from a previous year will not transfer if the file is password protected.

Here's how to transfer:

You have to use TurboTax Online on your computer to do this transfer (it won’t work from a mobile device/mobile web, or in the mobile app).

Sign in to TurboTax Online and start a new return, or continue a return you already started by selecting the Take me to my return button.
After the screen has loaded, select the TaxTools drop-down menu in the left-hand column, and then select Tools.
In the pop-up window, select the Transfer last year's TurboTax return from your computer link. (If you don’t see this option, you may need to Clear & Start Over.)
Follow the on-screen instructions to finish transferring your file.

That is exactly what that answers addresses. Click on the second blue line within it.