Revealing mistake: When Indy is in the archive room cornered by the thugs, there are a couple of wide shots where it's definitely not Harrison Ford, but a stuntman with a mask resembling his face. The wide shots cut to close-ups of the real Harrison, and one can see the big difference between both. According to the making-of videos, several stunts were played using this technique.
Continuity mistake: When Helena and Indy are cornered in the archive room, Helena—in the background—rushes upstairs. A shot later, she is metres behind, arriving at the steps again and repeating all previous movements.
Answer: It was never explained and seems impossible that anyone could survive such an impact. My own thought was the scene was deliberately exaggerated to appear as if Voller was killed in order to fool and then surprise the audience when he later turns up alive. I also thought it looked as if he hadn't aged. Voller may have been much younger than Indy, possibly as much as 25-30 years. When Voller reappears in the 1960s, he looks more like actor Mads Mikkelsen's actual age. The film should have made him look younger at the beginning. There's a lot of "suspension of disbelief" here.
raywest ★
It is also possible that he underwent facial surgery after surviving this.