https://www.reaconverter.com/convert/gtiff_to_ppm.html
Convert GTIFF to PPM – Batch Converter for Windows
Convert GTIFF to PPM without installing QGIS or writing ogr2ogr commands. reaConverter reads GIS data directly and works through whole folders in one pass.
for windowsconvertgtiffppmbatch
https://www.reaconverter.com/convert/gtiff_to_pgm.html
GTIFF to PGM Converter – Offline, No Upload
GTIFF to PGM offline, so survey, cadastral and client data never goes through an upload form. No file size limit and no per-file quota.
gtiffpgmconverterofflineupload