Start with the file type
The best smaller format depends on the file. Photos often shrink well as JPG or WebP. WAV recordings usually become much smaller as MP3. MOV videos are often easier to upload as MP4. Editable documents are often safer as PDF.
If an upload page also lists accepted formats, solve both problems at once: choose a smaller format that the destination explicitly allows.
Choose a format the upload form accepts
Do not only chase the smallest file. If the destination accepts JPG, PNG, PDF, MP3, or MP4, choose one of those common formats first.
Use JPG or WebP for images, MP3 for large audio recordings, MP4 for videos, and PDF for final documents that should keep their layout.
Check before submitting
Open the converted file, confirm the size is below the upload limit, and make sure text, audio, video, or images still look and play correctly.