JPEG to PNG: a working copy, not recovered detail
A document template or receiving system asks for PNG, but your only copy is JPEG. Conversion can solve the format requirement. Improving the image is a different job: a compatible file is not necessarily a more detailed one.
What does the new copy preserve?
PNG stores image data with lossless compression. When starting from JPEG, however, the starting point is the image that can be decoded now, not the camera data before compression. Saving that image as PNG does not restore discarded detail.
Halos around lettering or small block-like artifacts can remain. Compare edges at the same display size; changing the extension is not a restoration process.
References: W3C — PNG specification: lossless storage and transparency
When is a larger copy worthwhile?
A PNG copy is useful when a workflow requires that format, or as an intermediate copy that will not introduce another lossy compression step on its next PNG save. Keep the original JPEG so you can return to it and compare results.
A PNG made from JPEG can be larger. That does not mean the converter added detail. If the only aim is sending the same picture and the recipient accepts JPEG, there may be no practical reason to convert it.
References: Google WebP — File size, format features and limitations
A new extension does not create a transparent background
Moving a picture with a white background into a format that can carry transparency does not remove the white area. Background removal is a separate edit. Lettering drawn into the picture does not become editable text either.
ToolRoof handles a still-image conversion here, not resolution enhancement or layer recovery. Check orientation and dimensions after downloading. Keep the original when capture metadata or color management matters: the converted copy is not an archival duplicate of every original property.
Example: a picture for a document template
You have a 1200 × 800-pixel JPEG and a template that accepts PNG only. Without cropping or resizing, the image grid remains 960000 pixels. This checks dimensions; it does not establish that new detail has been recovered.
Open the PNG in the destination template and inspect small lettering and orientation. If the lettering is blurred in both versions, seek a clearer original instead of repeatedly converting. Keep both files rather than overwriting the source.