Looking for a ffmpeg command that would convert IphoneX HEVC 4K 25fps videos to any format that will work on PC - PremierePro and keep it lossless.
Conversion always will involve some loss, only copy (-c) is lossless.
There are some suggestions for "lossless" encoding in this Q&A, but I can't test them on my phone. There are UpVoted and have an accepted answer: the intermediate and output file will be uncompressed, and huge.
You can minimize loss when converting your existing videos by choosing an excessively high bitrate, since it's a cellphone video it will be of a limited length and thus only expand so much.
The ffmpeg H.264 Video Encoding Guide claims to offer lossless encoding using the
-crf 0 command. It also mentions that you should expect incompatibility problems with some programs.
The section: Choose a CRF value says:
The range of the CRF scale is 0–51, where 0 is lossless, 23 is the default, and 51 is worst quality possible. A lower value generally leads to higher quality, and a subjectively sane range is 17–28. Consider 17 or 18 to be visually lossless or nearly so; it should look the same or nearly the same as the input but it isn't technically lossless.
Using '0' isn't so much a "conversion" as it is a decompression and wrapping up the uncompressed video, the so-called converted video isn't recompressed.
You can correct the problem at it's source (in future) by:
On your iPhone, go to Settings > Camera > Formats > change "High Efficiency" to "Most Compatible".