Not necessarily tax specific but general PDF form filling and signing which is common and basic. Not every form has an associated app. Just had to sign my insurance which is web/pdf based. For tax, app is inferior to the web version of TurboTax anyway but on iPad which doesn't have a full desktop browser you have to settle for app.
PDF Expert works fine for me with most PDF forms (including IRS Form 1040, it seems).
Only exception I've encountered thus far was USCIS Form N-400 for which I needed Adobe Acrobat Reader for desktop to even render. Mind, I believe USCIS has an online application form which works on mobile devices now.