ZIP JPEG, PDF and MP4 Files Without Recompressing
Choose “Store without recompressing” for JPEG, modern PDF, MP4, ZIP and similar files whose own formats already compress their payload. ZIP still adds a filename, CRC-32 and directory entry for each file, so the archive may be slightly larger than their combined sizes; its value is one portable container, not another lossy media conversion.
Up to 200 non-empty files, 50 MB each and 250 MB altogether. Their bytes stay in this tab.
ZIP storage method 0 copies each source byte and adds only archive structure and checksums.
Running DEFLATE over already-compressed media usually spends time for little or no saving and can make the ZIP marginally larger.
Storage is not the same as leaving the files loose: one ZIP still preserves names, boundaries and integrity checks in one artifact.
How it works
How it is done
- Check that the selection contains 1 to 200 non-empty files, no file is larger than 50 MB, and their combined size is no more than 250 MB.
- Reduce every selected name to a flat basename, remove path separators and unsafe control characters, protect reserved Windows names, cap the UTF-8 name length, and number collisions without regard to letter case.
- Start one ZIP entry per file. Store mode copies the bytes with ZIP method 0; Fast, Balanced and Smallest use method 8 DEFLATE at levels 1, 6 and 9 respectively.
- Feed each source into the ZIP writer in 512 KB pieces. The writer calculates CRC-32, emits the entry records and reports progress while the worker yields between pieces so Cancel remains effective.
- Give every entry the fixed ZIP date 1 January 1980 and portable read/write attributes, then finish the central directory and offer the completed bytes as the requested .zip download.
What it assumes
- Selected files become flat entries at the root of the archive. Folder structure, source paths, comments and source modification times are intentionally not copied.
- Store mode preserves every source byte but does not promise that the ZIP itself will be smaller. ZIP headers, filenames, checksums and its central directory add overhead.
- Fast, Balanced and Smallest are three effort levels for the same standard DEFLATE method. A higher level can take longer without producing a meaningfully smaller archive, especially on JPEG, MP4, PDF, ZIP and other already-compressed formats.
- CRC-32 catches accidental corruption when an entry is extracted. It is not a cryptographic signature, authentication mechanism or defence against deliberate modification.
- This tool does not encrypt entries or add a password. The resulting archive is an ordinary unencrypted ZIP whose filenames and contents are available to anybody who has the file.
- Source bytes and the growing archive are held in the browser worker, which is why the per-file, combined-input and output limits are explicit.
Common questions
Does Store without recompressing reduce JPEG or MP4 quality?
No. The original file bytes are copied into a stored ZIP entry and are not decoded or encoded. Extracting that entry reproduces the same bytes and therefore the same file checksum.
Why can a stored ZIP be larger than the files inside it?
A ZIP needs a local header, filename, data descriptor and central-directory record for every entry plus an end record for the archive. With no compression saving, that structural overhead makes the container slightly larger.
Sources
The full method, worked example and every assumption behind this figure are on ZIP File Creator.