Hi @Kitz,
Could you please share a bit more about your scenario?
How do you convert a single image to a PDF on your side? Using custom connector?
I suppose that you save the image into a HTML file firstly (using Microsoft Flow), using <img> tag to collect the image, then convert the HTML file into a PDF file, is it true?
If your solution just as I mentioned above, you could consider take a try to concatenate (using Concat function in PowerApps) all image content within your Gallery control into a single one string value with a specific separator (e.g. '#'), then pass this concatenated string to your flow, and use the Split function to split this concatenated string into an array.
Please check the following blog for more details:
https://www.techmikael.com/2017/05/saving-collection-of-images-from.html
More details about Concat function, please check the following article:
Concat function
Best regards,
Kris