Bohemia provides the DayZ Tools Launcher (includes Object Builder).

If you have searched for this phrase because your tool crashed, follow this exact troubleshooting guide.

If you have 100+ legacy P3D files from DayZ Mod (Arma 2) and want to import them into a modern DayZ UPD server, use this batch wrapper for Mikero's tool:

# Batch Debinarizer for DayZ UPD 1.25+
$folder = "D:\Legacy_Models\"
Get-ChildItem $folder -Filter *.p3d | ForEach-Object 
    Write-Host "Processing $_"
    # The -W flag forces the latest DayZ workspace revision
    & "C:\Tools\P3DTool.exe" -unbinarize -W=DayZ $_.FullName
    if ($LASTEXITCODE -ne 0) 
        Add-Content -Path "failed_models.txt" -Value $_.Name

Before fixing the error, we must understand the culprit. The .p3d file format is the proprietary Bohemia Interactive mesh format used since the Operation Flashpoint days. In DayZ Standalone:

Since DayZ 1.0, BI moved toward encrypted or compiled binaries to protect assets. However, when modding, you often find legacy .p3d files that are "binarized"—compiled into a binary format your extraction tools cannot read.