Loseless conversion from IphoneX HEVC for PC - PPro
Conversion always will involve some loss, only copy (-c) is lossless. 

There are some suggestions for "lossless" encoding in this Q&amp;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".