When i create a new document which is 1000 x 1000 mm i can save as a photoshop document and also save as a pdf.
If I wanted a document 1800 x 2500 mm it will save as a large format document but no option to save as a pdf. Hence there must now be a size limit on saving as pdf. is this just in width or length or in size of the file.
In an earlier version of Photoshop CC i have saved 914 x 5000 mm documents as pdf. Since Photoshop CC 2018 this is no longer possible.
Is there alternative way of saving these large documents as pdf so that they can be printed?
As part of your CC subscription, you can install and use that version of Photoshop that saved your larger PSDs to PDF.
I agree this is not the best answer, but you should be able to finish the job.
Do you have Illustrator or InDesign? You can save your artwork as PSD, PSB or TIFF, place it into an AI or INDD and then save to or export to PDF from there.