CloudConvert is a online file conversion utility. More than 200 different formats are supported:
✓ audio (MP3, AAC, FLAC...)
✓ video (MP4, AVI, MPG...)
✓ document  (PDF, DOC, DOCX, ODT...)
✓ ebook (MOBI, EPBUB, CBC...)
✓ archive (ZIP, RAR, 7Z...)
✓ image (PNG, JPG, SVG...)
✓ spreadsheet (XLS, XLSX, ODS, CSV...)
✓ presentation (PPT, PPTX, ODP...)

Some example conversions:
* extract ZIP, RAR etc directly on Google Drive™ / Dropbox™
* merge multiple PDFs, JPGs, DOCs into one PDF
* PDF to DOC 
* AVI to MP4 
* M4A to MP3
* MOBI to PDF
* DOCX to ODT
* PNG to JPG
* and many many more. see https://cloudconvert.com/formats for details.

Most formats offer advanced options. So it is possible to specify custom settings like codecs, bitrate, resolution etc. for video conversions.

It is possible to use Google Drive™ or Dropbox™ both as input or output destination for your converted files. This makes it possible to start a large video conversion, leave the app and just wait until the file appears in your Google Drive™ / Dropbox™.