I also have more informations on the problem it look like even if the Azure function should be a thread whit only one pdf to convert. It look like Process when multiple Azure Function are trigger is creating some Memory Allocation Issues whit MIcrosoft.WindowsAzure.Storage.
Exemple of Memory Allocation Issues:
MemoryAllocationIssuesExemple.jpg (236.1 KB)
And I think that the memory issues is causing the processing to use more and more RAM the more you have Function trigger at the same time (to much for the azure function environment).
Exemple of a Process RAM of one pdf: It need approximately 0,3 of RAM
ProcessByteOnePdf.PNG (6.6 KB)
Exemple of a Process RAM of multiple pdf: Process are going up to approximately 3G of RAM for 9 pdf
ProcessByteMultiplePdf.PNG (6.0 KB)
For the pdf sample you can use whatever content but this is example:
SamplePdf1.pdf (48.9 KB)
PfdSample2.pdf (66.6 KB)