Learn Farsi: مرد جوان یک پلاکارد سنگین را بالا برد.

At the B2 level, you will start seeing this combination to mark a 'specific indefinite' object. The placard is indefinite because it is newly introduced to the listener (hence yek), but it is a specific, tangible object the man is interacting with. Farsi uses râ here to highlight this specific, concrete focus.

Is پلاکارد a loanword? How do Iranians pronounce the 'pl' consonant cluster at the beginning?

Yes, it is a loanword from French. Since native Farsi phonology does not allow two consonants at the very start of a syllable, speakers naturally insert a short vowel sound between them. In everyday speech, it is pronounced pelâkârd.

The verb here is بالا برد (from بالا بردن). Could we have used بلند کرد instead?

Yes, both are perfectly natural here. بالا بردن literally translates to 'to take up' and is great for hoisting or raising a sign, a hand, or a flag. بلند کردن (to make tall/high) is also frequently used for lifting physical objects off the ground.