Allowed memory size exhausted

[RitzenthalerF]

Hi there,
I'm trying to implement PDFMerger and I'm getting the error below:

PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 20480 bytes) in /web/Classes/PDFMerger/tcpdf/tcpdi_parser.php on line 898

This doesn't occur for every merge, mostly 1-3 files work fine, but as soon as I run it on 5-20 files, I'm getting this error. I couldn't figure out what causes this error, because mostly I'm merging files below 1MB. Is this issue known in any way?

Thanks a lot in advance!

Best wishes,
Frederik