Links between PDF documents break/behave differently when opening from Sharepoint

I am using Adobe Acrobat Pro DC to create a document which needs to be distributed to customers. The current version of the document is a single PDF with a number of attachments. Unfortunately, the document is now too large for the email and web portals limits used to distribute the document, which causes problems. I am therefore reengineering the single pdf document into two separate pdfs (Part1 and Part2), with links between them working as if it were a single document.  The links I have made are of four main types (all created using the "Go to a page view" option):

1. A link in Part1 to a location in Part1,

2. A link in Part1 to an attachment in Part1,

3. A link in Part1 to a location in Part2,

4. A link in Part1 to an attachment in Part2.

I have managed to do this and everything works fine from my local machine (with both files in the same folder). If I copy the two files to a different folder on my local machine, everything still works fine. Great.

However, when I upload the two files into Sharepoint and open it from there:

• If it opens in the Acrobat app then link types 1 and 2 work, but link types 3 and 4 do not. This is because, for example, where on my local machine link 3 would be to "../Part2.pdf", when opening from Sharepoint, it becomes "../Part1.pdf/Part2.pdf", which does not exist in that location.
• If it opens via the browser, link types 1, 2 and 3 work (after prompting the user before opening the clicke link's URL) but link type 4 does not.
• If I redownload the two files from Sharepoint to my local drive, everything works again as normal.

So my questions are: why? Is there a way to obtain consistent link behaviour in opening from a local drive and from Sharepoint? And if so, is it something I can do when creating the PDF, or is it down to some Sharepoint/PDF setting on the admin/or user side? Thanks in advance for any advice.